Z will cast Synaptic Static, trying to cover as many of the Goblins within its 20ft radius sphere as possible.

Damage 32 Psychic, or half with a DC16 INT test.

After a failed save, a target has muddled thoughts for 1 minute. During that time, it rolls a d6 and subtracts the number rolled from all its attack rolls and ability checks, as well as its Constitution saving throws to maintain concentration. The target can make an Intelligence saving throw at the end of each of its turns, ending the effect on itself on a success.

Randall:

Randall keeps guiding the increasing flow of water down the tunnel at the goblins and away from his friends.  As an afterthought, he conjures a glowing mace and sends it at a goblin headed his way.

Spiritual Weapon:  Attack: 30 Damage: 12

Pepin creates a tiny cannon mounted on Randall's shoulder and wards him with magic.

The goblins struggle to regain their footing in the flow of water and make any headway toward the party members.

Petcan opens the pipe he is working on and more water cascades into the room.

Zenodor uses magic to make a psychic explosion that kills 11 goblins.

Randall continues to guide the flow of water and summons a spectral mace that nearly takes a goblin's head off.

(Tempe is up.)

(How many Goblins are there in total? Was it 12 originally, with 9 dead - so 3 remaining?)

Tempe though sure his spell wont work once the goblins have to swim cast spike growth to give his friends a break in defending. He points to a point "Don't pass that point" he says as he casts the spell.

(Yes 9 dead and 3 remaining from squad one. 1 of the 3 remaining is barely alive.)

After Tempe places his spike growth spell a 2nd goblin squad enters the pumping station.

68

Initiative for squad two: 15

The 2nd goblin squad does not fall prone but still has to deal with moving through the water flow (difficult terrain).

Initiative for Round 2:

Pepin
Goblin Squad One
Petcan
Zenodor
Randall
Tempe
Goblin Squad Two

Oh yeah... Mignon does not fight but at the end of round 1 he decides to start climbing up the access shaft.

"I'll check and make sure we can get out this way!"

So Pepin is up?

Pepin uses his action to open the pipe again. While his bonus action is used to activate his cannon.

Targetting a 15ft cone from Randall (he has 15ft movement to move around on his shoulders so should be able to target any direction) with as many goblins as possible. DC17 dex save for 12 fire damage.

4 separate saves for the 12 goblins, each affecting 3 of them:

DEX save 1: 3

DEX save 2: 15

DEX save 3: 22

DEX save 4: 16

Pepin's cannon shoots off a blast of fire like a flame thrower showering the new squad of goblins that just entered the pumping station, leaving them screaming and writhing in the agony of the flames. The flames are quickly extinguished by the water and don't do as much damage as they normally would.

Pepin manages to finish opening his pipe and now only 1 pipe remains to be opened. Water is flooding into the room and being directed by Randall's spell to flow out through the tunnel you all entered from.

Petcan continues opening pipe

Zenodor casts Synaptic static once more, setting the focal point to target as many Goblins/groups in the area of effect as he can (while not targeting his friends).

Psychic damage: 32, with half damage on a DC16 INT save (and the muddled thoughts effect for a failed save)

Z then starts moving towards the ladder and up it, as far as his movement speed allows. Can Z see anything at the top where Mignon went ahead?

(Being a Warlock, that's Z fresh out of spell slots)

(and the spike growth?)

Randall:

On his turn, Randall continues to guide the building water torrent around his friends and through the goblin foes.  He starts backing up towards the exit and sends his Spiritual Weapon to strike at the nearest Goblin.

Spiritual Weapon:  Attack: 21 Damage: 7

(Also, as we keep opening the water more and more - is it making it harder for the Goblins? I'd expect that once all are fully open, they will be flushed out/drowned)

Zenodor's psychic blast again kills goblins, 10 this time around.

(Almost forgot.) Tempe's spike growth deals with those that moved to melee. (They actually die from movement alone and don't get to dash.)

One more round and Petcan will have opened the last pipe.

Randall's floating weapon strikes at a goblin being washed out of the tunnel, many more are bobbing and swirling their way out of the pumping station. It is hard to judge if the strike lands as the goblin sinks beneath the surface.

With the goblins beginning to float away pepin heads towards the ladder, rope in hand to throw to anyone who might need it.