(Good thing Brocc was there for that sneaky damage :)

Springing back Jak launches an arrow that narrowly misses Broccs shoulder and passes right through the psuedopod. At first it seems like nothing happened, but then the psuedopod starts to dissolve and an oily grey substance starts to bubble up and dissipate through the water. The Ooze is Dead. (long live the ooze. haha, j/k there's not another ooze:)

Blinking in surprise at the success of his shot, Jak straightens up, striking a heroic pose. "Ha! Nothing to it! Thank you for luring the beast out, Master Brocc. All according the master plan of I, Jak the Master Sharpshooter!"

After Jak is done talking, Johnson walks up to Jak and says to him, Great shot Jak my friend, you too Brocc, but next time leave some of the baddie for me eh. As Johnson playfully punches Jak's arm. 

After everyone's adrenaline has lessened you all continue on. You can see the passageway bends to the right about 40' ahead obstructing your view. 

Dm stuff

 You all make it round the bend without further incident. Ahead of you is more passageway. You can see to it's end though now. At it's end it turns sharply to the right. About 20' before that you can see another passageway going off to the left. You do notice something odd (passive perception)

There is a disturbance in the water going from the left passage to the right. Ripples. Just a few against the walls of the passageway. Then while you watch more ripples moving out from a specific point. Like a foot has been very slowly moved into the water.

 Jak grimaces at Johnson's "playful" punch, doing doing his best to pass off the expression as a smile. (Deception check- 18) "Right, next time, of course" he hisses through gritted teeth. "Shall we continue?"  He gestures down the hall, rubbing his sore arm quickly when the party looks away.

Symon Twist Perception   23

"Indeed t'was a foin shoot you be taken' thar Jak.  Almost as good as Oi moight bes doin a'ter a couple o' tankards, heh, heh," he chuckles to show he is joking and is impressed.

"Ho," he whispers. "bes ware, they bes sompin' oop ahead."

Grumbling, Brocc finishes cleaning in lingering gunk from at least the parts of him not touching the water along the way. So, when the group arrived, and someone mentioned something about a something up ahead, and already flustered Brocc glares in said direction. "Don't worry, gentlemen. As always, I have the perfect solution to all of life's problem. Observe what a few flicks of the wrist and the right words will reveal in a matter of seconds."  Without waiting for much of a response, Brocc traces another circle in the air.

Trailing behind his fingers is a line of fire that, upon completing the circle snaps into a single complete point and flies off just above the water with the utterance of "Firae!"

Action(more or less): Cast the Cantrip Fire bolt.
To Hit: 10/25(dis?) For: 14 fire damage.

(I can't see what you were casting but yes, attacks against unseen targets are as disadvantage. This creature is invisible)

Brocc launches a spell to where he thinks the creature is based on the ripple it just left but it slams into the back wall, apparently having missed whatever has left the ripple behind. 

Initiative incoming

Initiative:

Brocc: 12

Johnson: 11

Jak: 5

Symon: 6

Invis thing: 12

Initiative order: Broc, Inivis, Johnson, Symon, Jak.

(as it stand right now you believe the invisible creature is 35-40 feet away somewhere near where the passage turns to the right)

(It was fire bolt. and I have since fixed it. Sorry about that.)

(Also once again, Brocc somehow wins initiative! lol!)

Brocc huffs and makes a show of rolling back his sleeve as he'd begun marching to the creature. "Alrrrrighhty then! Think you can brush off my magics, do you? Well I'll show you whipper snapper a thing or two! Quote so! Hestaga!" He snaps his finger. After which his form almost appears to vibrate before the gnome is just suddenly off like a gunshot. Although his own splashing around might make it difficult to stop quite in time, he had a good head for distance, and so trusted to hit true upon drawing Hammerdinger at the end of his race forward!

Oddly enough, he drops the hammer with the second swing, appearing to have overextending himself just a bit too much!

Bonus Action: Cast Expeditious Retreat on self, and immediately move 30ft closer to the creature.
(For the curious, I took the Squat Nimbleness Feat, so he has 30ft base movement like all you other talk folk)

Movement: Further into potential melee with it.

Action: Attack w/ Extra Attack.
Hammer Time - To Hit: 25/8 For: 11 non-magical bludgeoning damage to invisithing.
Hammer Time(Again!) - To Hit: 21/8 For: 10 non-magical bludgeoning damage to invisithing.

Or(if in difficult terrain)

Action: Cast another Fire Bolt after moving as closest as possible.
To Hit: 13/20 For: 14 non-magical bludgeoning damage to invisithing.

(Yeah, it is kind of odd you get first init since you have the lowest modifier)

(So I want to make sure I've got this right, based on your options)

It is difficult terrain still with the water on the floor. Brocc bonus actions expeditious retreat, but is only able to move 15' closer to the invisithing. He casts a fire bolt to where he thinks invisithing is and for a moment you think you see some kind of shimmer just before the bolt dissipates against the back wall. (You can't use expeditious this round though because bonus action is used but it is noted for the next 10 minutes!)

(This is potentially a really tough encounter and what's crazy is it's listed as a random encounter! I guess at least if it comes up again you'll have an idea of how to deal with it though :))

This will bring us to Invisithing...

After watching your attack again hit the back wall you see something odd on the water. Almost as if a light wind were blowing in the tunnel traveling towards you very quickly as something (attempts to) slam into Brocc twice.

Attack: 21 Damage: 11

Attack: 25 Damage: 8

(As part of the casting of the spell, as well as on subsequent turns, one can dash with it. But, eh, I am happy to not have Brocc ran too far ahead of everyone else just as well. XD)

#1 Conc. Save(DC 10): 26.

#2 Conc. Save(DC 10): 15. 

(Ahh, yup. Missed that first part of the wording. Sorry about that)

That brings us to Johnson

After seeing his companion get hurt Johnson flies into a barbarian rage, Recklessly attacking the invisithing with his Versatile Warhammer.

(since i have disadvantage on attacking the invisithing but my reckless attack gives me advantage do I just roll normally?)

(Correct Johnson. They cancel each other)

Symon will cast Bless on Brocc and Johnson. "Feel th' burn me boyo's!"

First Attack: 9 Damage: 9  + 2 to hit

Second Attack: 20 Damage: 6

Both Johnson and Broccoli, you can add 1d4 to your attack roll -from the Bless.

(If were sticking to initiative order only Jak will benefit this round but Johnson doesn't need it though (AC14 invisithing) Unless there's some ability I'm not aware of?)

Both of Johnsons swings connect with something but they both seem to pass through too easily and don't produce the damage expected causing 12 dmg

That brings us to Jak.