Shere thinks to himself "This is no bueno"  He casts slow (Wisdom DC15) on the dinosaur which reveals himself! Yelling "Hey! Cup cake!" Then waiting long enough, making sure the giant creature follows, he uses his cats grace to move 60ft into the jungle

That's all Rivon needed to see.  His arrow flies toward the big lizard with the useless arms.

Sharpshooter Bow of the Blindside + Sneak Attack with advantage from hiding:
Attack: 9 Magical Piercing Damage: 33

Before it hits, he's already moved to a new shrub for cover. 

Stealth (to not be seen): 16

Quote from Mr_Grim123 >>

Dex save: 13

 Although Toot got blown aside from the T-Rex's breath and tumbles backwards a few times, he's still able to maintain his flight, and quickly ducks around a tree. Being so small, the T-Rex doesn't even notice him.

As the T-Rex is chomping on the deceased Halfling, Three starts pantomiming to those she can see (Toot and Fujin) to see if she should attack or try to flee. Shere pops out of the bushes casting a spell at it and yelling for it to follow him as he starts to lead it away from the group.

T-Rex Wisdom Save: 16

Unaffected by Shere's spell, the T-Rex starts to follow Shere for about 20 ft. Then, Rivon also pops out of the bushes from the other side, firing an arrow at it and hitting it in its left haunch. The T-Rex lets out a loud roar, then turns to head where Rivon was hiding.

INITIATIVE:

Toot - 21 <===
Rivon - 21
Three - 18
Shere - 16
T-Rex - 16
Fujin - 10

Toot Toot flys up so he remains out of range of the lizard and casts Faerie Fire (Dex save of 14) on the lizard. Once cast, he backs up a bit more.

Rivon peeps out from behind his shrub and sends another magically conjured arrow toward the hulking beast.

Sharpshooter Bow of the Blindside + Sneak Attack with advantage from hiding or Faerie Fire (assumed.  Drop 2nd roll if both failed):
Attack: 20 Magical Piercing Damage: 30

Realizing then that a shrub offers little actual protection against a beast of this size, Rivon darts behind a larger tree, which he hopes will give him some protection should the lizard try to eat him.

Stealth (to not be seen): 30

(OOC: My "to not be seen" stealth rolls are indicative of sight based stealth.  When I use "while moving silently" it's more about staying quiet.  Those rolls will have advantage due to my Boots of Elvenkind.  I just wanted to explain the difference as otherwise it seems weird.)

Having lost the initiative in her own surprise at the turn of events, Three is yet quick to recover in drawing forth her bow. Though realizing shooting while up in a tree is a touch awkward, she scrunches up her face and dug deep until from a disguised flap on the back of her pants sprouted a fairly prehensile and scorpion-like tail. "Oh yea~ That's the stuff." She murmured and shuddered a little in relief, before then having the new limb coil about the tree as she lined up shot after shot for the head of the giant lizard. 

Bonus Action: Enter Rage(Tail Form). 

Action: Multi-shot at T-Rex.
(Short Range, Lethal)Longbow - Attack: 22 Damage: 10 of non-magical piercing. 
(Short Range, Lethal)Longbow - Attack: 25 Damage: 5 of non-magical piercing. 

(OOC: Only 1st attack at adv. from attacking from hiding. So ignore 2nd roll in attack 2 if Faerie Fire wouldn't apply)

Shere casts slow once more.

Toot Toot flies up above the T-Rex and tries to make it even easier to see.

T-Rex DEX Save: 1. The T-Rex is now glowing and is much easier to see.

Rivon fires another shot from behind the bushes (hitting it yet again) and then darts behind a tree to make it harder for the T-Rex to eat him. Three produces a tail from her pants and uses it to stabilize herself in the tree as she fires her longbow at the T-Rex hitting with each shot. Shere again casts a spell in the attempts to slow down the T-Rex.

T-Rex WIS Save: 2. The T-Rex seems to moving a little more sluggish than its normal sluggish speed.

The T-Rex buries its head in the bushes where Rivon was hiding, but does not find anything there. It roars as three more arrows strike it from different positions and looks around to find the source of the pain. It sees Three in the tree and moves over towards her but is not quite able to get close enough to bite. Three can however smell the rotting flesh that is still stuck and dangling in the T-Rex's jaws.

INITIATIVE:

Toot - 21 
Rivon - 21
Three - 18
Shere - 16
T-Rex - 16
Fujin - 10 <===

(lol this round is starting to rhyme) 

Fujin, not wanting to feel useless in this fight, inhales sharply before sprinting from around the tree he was hiding by, barely reaching the T-Rex with his stubby legs.

Action: The dwarf charges right next to the dinosaurs leg, a translucent wavy energy manifests around his warhammer as he brings it in for a mighty swing. All of the gathered energy blasts out instantly at the moment of impact. His intent is to draw the attention of the beast solely to himself if he can.
To Hit (Goading Attack): 11 (T-Rex has -2 AC from slow)
Bludgeoning Damage: 14 + 3 (Goading Attack) + 1 (Force Damage)
If it is hit, the T-Rex must make a DC14 Strength Save or be pushed 15ft away from the tree where Threesa is perched.
If it is hit, the T-Rex must make a DC15 Wisdom Save or else it has disadvantage on all attack rolls against targets other than me until the end of my next turn.

Edit: I'm stupid and also level 5, Multi-attack roll.
To Hit: 15
Bludgeoning Damage: 10

Fujin rushes out of the bushes from the tree he was hiding behind and strikes the T-rex on the leg. 

T-Rex STR Save: 12 vs. DC 14.

T-Rex WIS Save: 15 vs. DC 15.

The T-Rex gets pushed back 15 feet and lets out a fierce roar, it then turns its head to the source of the pain.

INITIATIVE:

Toot - 21 <===
Rivon - 21
Three - 18
Shere - 16
T-Rex - 16
Fujin - 10 

Toot?

Sorry everyone, last few days have been hectic.

Toot Toot eyes widen as he grins and says "Fun Fun Fun Fun Fun! I love this place!" He speeds through the air acting like on an invisilal runway and does a cartwheel while flying (8) to look cool while doing it, raising his crossbow while over the lizards head and fires (14) to hit and (18) if successful. He finishes the 'flip' after passing the head and 'lands' on the invisible roadway raising his hands in the air "And the crowd goes WILD"

It wasn't that long ago that Rivon considered Three to be the strangest person he knew.  His new companions have completely shifted his views on this point.

Trying not to be overly distracted by Toot's antics, Rivon lines up what must surely be the killing shot on this tall, scaly SOB.

Sharpshooter Bow of the Blindside + Sneak Attack with advantage from Faerie Fire:
Attack: 11 - Magical Piercing Damage: 27

Out of habit, he dives into a new bush before the arrow even arrives at the target.

Stealth (to not be seen): 13

Despite the oversize terror mere feet away, Three looked on excitedly at the beast... until its breath washed over the chimera, prompting her to scrunch up her face and wave a hand to try and waft the stink away. "Who-wee~ I haven't smelled a breath that bad since Big Sis te--" But the rest dies in her throat watching the oversized creature suddenly shoved away. Confused, she'd look around for the source, and in seeing Fujin at its feet, her eyes went wide at first before relaxing into something like... admiration? 

 Likely hard to say for anyone in the heat of the moment. But soon enough, she focuses her attention again on the oversized lizard to try and plug a few more arrows closer to its eyes.

Action: Multi-shot at T-Rex.
(Short Range, Lethal)Longbow - Attack: 20 Damage: 8 of non-magical piercing. 
(Short Range, Lethal)Longbow - Attack: 28 Damage: 14 of non-magical piercing. 

(Note the the rex's AC has a minus 2 to it. And after you attack you should really run away from it)

"Yo! Amigos, should rreally get away frrom it while you can!"

Shere on his turn cast Dissonant whiskers  at 2cnd level but holds the spell till the end of the giant proto dragons turn

His song, a song of pain and the monsters worst fear (Hunted by something ten times bigger than it.) as psychic whiskers surround the creatures mind.

DC15 wisdom save 11 (Note if it saves on the Slow and it fails on the whiskers those close enough to it get an aoo on it.)

Toot Toot flies around the T-Rex and tries to gracefully perform a cartwheel in mid air, which looks more like a toddler trying to learn how to cartwheel. He then fires a bolt at the T-Rex's head. Rivon then takes another shot at the T-Rex before hiding somewhere amongst the bushes again. Three also raises her bow for some more hits, and Shere tells everyone that they should probably move away from the T-Rex as he starts singing at it.

T-Rex WIS Save: 20

The T-Rex then lashes out at Three attempting to bite her.

T-Rex Bite Attack: 25 If Hit: 41 piercing damage and an Athletics/Acrobatics Check to avoid Grapple.

INITIATIVE:

Toot - 21
Rivon - 21
Three - 18
Shere - 16
T-Rex - 16
Fujin - 10 <===

(Err it didn't work and don't forget the dissonate whispers)

Reaction: Tail Swipe vs Bite, raising AC by 3. If 7 or higher attack turns into a miss.

If not a miss, Athletics: 16 vs grapple(?). 

.