Mordekai is unable to hellish rebuke, as the wizard is too far away. But his dispel magic has the range it needs.  He casts it at 5th level....hoping to dispel the invisibility....which he does!

Everyone sees the wizard appear, including skeletons!

Even with the wizard throwing up an invisible protective shield, and half the arrows bouncing off the shield below, several arrows find their mark and slam into the wizard. He is the same looking as the others you were ambushed by....pale with dark hair, an elf.  He clutches his side as he hisses in pain. 

Con saves to maintain spell : 15. 10.  9. 14 

The invisible wall in the middle of the place vanishes abruptly, and the frog creature that hasn't yet had a chance to stand up, falls another 20 feet to the ground below with a hard thud. 7 falling damage.

The wizard meanwhile growls in pain and casts another spell from the safety of his elevation. Mordekai immediately sees another invisible wall form, though this one is up high around the wizard, and forms a box around him. There is only a tiny gap between two edges, and from where he stands now, he has full cover from everything...as he's not near the gap.

The Slaad, finding itself surrounded by a veritable army of pointy arrow wielding, stands up and looks around, and attempts to cast a spell! (counterspell anyone?  If so, please accompany it with a roll.) 

Meepo's counterspell... 10

Meepo's counterspell attempts to stop the Slaad and does, the spell fizzling. Seeing the small kobold he stands up, and moves that way, though can only get part way towards the archway. 

Everyone's action!

Assuming there's a gap in the wall that Mordekai can identify that's big enough for any of us to fit through, Mordekai goes over to Lionel, and says, "Push the wizard down to the bottom!"  Then he'll put a hand on Lionel and cast Dimension Door to take them both inside the invisible wall space, next to the wizard.  He points to the gap in the wall, and says, "That way! And you're safe to jump down."

(If the gap isn't big enough for someone to fit through, Mordekai will just cast 2 Eldritch Blasts at the Slaad.)

(And if everything goes to plan and Lionel does jump down on his turn, Mordekai will cast Feather Fall on both himself and Lionel as a reaction)

Ulthan grumbles about mages and their spells again as he fires two arrows into the giant frog wondering what it tastes like.
Arrow1: 9 Damage: 18
Arrow2: 21 Damage: 23

Forgot an impossible concentration save (DC19): 19

The strange creature was now surrounded by the skeletons and Lionel now could see the wizard that controlled that thing. He flew up, then disappeared on that side of the wall to appear near the mage. Then he attacked:

BA: Misty steps from helmet

Attack: 13 Damage: 12 +10 (Radiant soul)

Attack: 12 Damage: 11

(So close!) Add to the first roll 2nd lvl smite: 19

Actually, use that Bardic Inspiration to save the Spirit shroud: 5 and add to the roll that hit 7, please.

Meepo calls for all of hiss skeletons to attack the frog-thing...

sk1 ... Attack: 7 Damage: 9
sk2 ... Attack: 19 Damage: 10
sk3 ... Attack: 11 Damage: 8
sk4 ... Attack: 24 Damage: 13
sk5 ... Attack: 7 Damage: 11
sk6 ... Attack: 13 Damage: 7
sk7 ... Attack: 19 Damage: 11
sk8 ... Attack: 10 Damage: 9
sk9 ... Attack: 24 Damage: 12
sk10 ... Attack: 14 Damage: 12
sk11 ... Attack: 20 Damage: 9
sk12 ... Attack: 19 Damage: 11
sk13 ... Attack: 18 Damage: 10
sk14 ... Attack: 8 Damage: 9
sk15 ... Attack: 7 Damage: 10
sk16 ... Attack: 15 Damage: 9
sk17 ... Attack: 21 Damage: 8
sk18 ... Attack: 23 Damage: 12
sk19 ... Attack: 6 Damage: 9
sk20 ... Attack: 24 Damage: 11

Big skeleton
Grave Bolt Attack: 18 Damage: 13
Grave Bolt Attack: 27 Damage: 12

He sends a blast of cold at it himself, as he moves back into the room, and calls for his zombies to come and block the doorway. ray of frost
Attack: 15 Damage: 13

(if it matters...)
Extra crit damage from the single skeleton that had a critical attack: 5

Lionel is flying near one wall.  Mordekai runs up the stairs, passing Ketch, and leaps off, his hand outstretched for Lionel. When he touches him the two of them vanish and appear way up the shaft near the wizard.  Lionel doesn't even need to fly or use his helmet, his target is suddenly right in front of him. He lashes forward with one powerful blow, the blade slashing through the shadowy figure with near lethal results, blood spraying across the inside of the wall.  10 Con save for spell DC 21.  The invisible wall around the wizard is short lived.  The blood spray falls freely down the shaft like so much red rain. Mordekai drops down from the spot he was on, and tries to keep his footing on the stairs he's suddenly landed on (Mordekai: Dex DC 15 to not slip off the edge) 

Meanwhile, at the bottom of the shaft, between Ulthan and the army of skeletons, fill the black skinned Slaad with arrows, thunking in one after another. The Slaad wavers and growls deeply but is still standing. A blast of cold by Meepo seems to have no effect on the creature. 

Ketch, your action. Looking around, your enemies keep moving. The Slaad is over 40 feet below you...though....a good jump from up here and you could probably land right on him. Otherwise, Lionel and Mordekai are above fighting the wizard who looks near death.  A few splatters of blood from above drizzle on your head. 

DC15 DEX save (with Lionel's aura): 14

Mordekai will cast Feather Fall on himself as he begins to plummet off the edge of the stairs. 

Lionel would try to catch Mordekai, but seeing him slowly and safely levitating, might as well stick to the finishing off the wizard.

BA - Ketch sends his Spiritual Weapon 20’ lower towards the creature.  Hopefully the thing will be dead by the time it gets there, but just in case…

Ketch himself will leap off the stairs and try to angle his shield between himself and the creature he is aiming for.  The goal is for the shield to absorb as much of the blow as possible while Ketch tries to plummet his axe into the creature as deep as he can… hoping to ride the axe down to the ground as it slices through the beast.  ((For the visual anyway, even if not for the damage…))

Axe Attack: 28 Damage: 10

11 for the Slaad. 

5 for Ketch. (wow, that works). 

Ketch lands full shield onto the Slaad, his axe driving down into the creature.  Both go tumbling to the ground amid a spray of blood.  

The Slaad looks about, full of arrows, but still moving.  He stands up and moves to one side of the shaft so that Meepo can't see him and casts a spell, vanishing. 

The wizard meanwhile looks at Lionel with a grim look, blood pouring from his body, and says, "The Queen shall have the marked one, there is no escape for you....she comes now with all of her armies to stop you from freeing the usurper!"  and with a hiss he casts a spell and vanishes. 

It's quickly clear the Slaad is not just invisible and is gone, as is the wizard.  

The battle appears over, the two formidable foes having fled. 

(out of initiative:  5340xp each) 

"Thanks for the warning," shrugged Lionel and then graciously flew down to the others. "The queen is coming with her full army," informed the others in case they did not hear and patted Laffa on the shoulder. "Feeling adventurous already? Chin up, that Princess might return your magic too? Who knows." "So, can we get to the other side already?" He came to the hole that was dug up by the skeletons and checked if everyone can move there (through the teleportation means if needed). 

Meepo sends his undead back into the portable hole, and looks at the state of the clearing his zombies had managed.

Staying alert, everyone returns to the room to check on the zombies as the last of the skeletons shuffle into the hole. It takes quite a while to clear enough rubble to make a hole through it that you can climb through, about 2 hours. Once they break through, turning it into something you can crawl through is trivial. There's no sign of any army during this two hours, though Meepo's undead flying cat spends a bit of time flapping around up above to keep an eye out. 

The air that seeps out through the hole is stale and dry. Though a layer of debris and dead insects coats the floor of this area, its contents remain largely intact. Four trestle tables, a side table set with what looks like cups, and two dozen chairs stand covered in dust. You see a side table that holds a tea service, covered in thick dust. Three ten foot wide archways lead to neighboring rooms. The one on the left has "CANDESCA" inscribed in stone above the archway.  As you move into the room with the tables you can easily glance each of the three rooms attached to this one.

The room on the left, under the "CANDESCA" inscription is filled with decaying desks, chairs, and lecterns. Three smooth, one-foot-diameter crystal hemispheres protrude from the ceiling, evenly spaced along the center of the room from east to west. A concave wall on the far side of the room has a steel-banded stone door in the middle of it. Three sharp-edged glyphs are cut into the surface of the door.

The floor of the room straight across is covered with filth and moldering debris, and reeks of untold generations of rats. A faded fresco on the back wall depicts several human, elf, and dwarf wizards battling a black dragon.

The room on the right is filled with moldering wooden furniture. Only three bunk beds remain recognizable as what they are, and all are close to collapse from rot. A door in the left wall has been smashed open, and a damp draft comes from beyond it.

"hrmpfh blind leading blind hrmpfh" says Ulthan mostly to the cat.

The time of clearing the way Lionel used for some rest (he felt like at least a couple of ribs were either broken or cracked) but then got into the next room with the others.

(HD: 37)

He looked around with disgust. "Would imaging anything with the word "princess" in it be a bit cleaner." Since the beholder's book-lair Lionel did not trust colourful pictures. Was that the real door in? So far, it looked like the room with the glyphed door and the crystals was the obvious way to go but who knows. "Oh well, should we check that door?"

Once inside the room, Ketch would ritually cast Detect Magic and walk around and see if anything catches his interest.  Otherwise he looks to Meepo - This is his show.

Much like the entrance room, Ketch begins to notice sections of glowing aura's all about the various rooms. 

The first room with the tables has one such section of conjuration magic emanating from the wall to the left of the entrance.

In the room to the right with the door, there is a similar section of wall to the right of the door. As well, the door and wall themselves are both emanating a powerful aura of abjuration and evocation. Inside a lectern he spots some magic, and glancing inside he sees a spell scroll of conjure fey.  

In the room straight on with the mural he spots divination and conjuration magic emanating from a spot in the tiled floor...and he also sees transmutation magic emanating from one part of the mural. It appears there is an actual wand stuck into an indent and made to look like part of the mural...and the wand is magical. There is also abjuration, conjuration, and evocation magic across the mural.  The left and right walls also have sections that are glowing with conjuration magic. 

In the last room with the decrepit bunk beads, he spots a section of the wall at the far end that is glowing with conjuration magic.