D&D 5E (2024) Need a recommendation for polymorphing enemies

[ECMO3]

So I am having the final boss fight this week in a 20th level campaign (my PC is a Fighter 12/Warlock 8). The end boss is a wizard Undead. Think Lich but substantially more powerful and two minions, one a Fighter type the other a Cleric type.

I have a scroll with a custom spell from the DM, it is a 10th level spell called Mass Polymorph. It works like a regular Polymorph but it can affect up to 10 creatures. I will need to roll a DC20 Charisma check to cast it, but I have a 26 Charisma and tactical mind and an inspiration, so I will probably make the Charisma check.

If I use the scroll I will turn party members near death into T-Rexes (or at least anyone that would benefit from it at the time) dishing out 100+ temp hit points to everyone. For the enemies I figure I want to go with something that has a swim speed but no walking speed and a lot of hit points with preferably a weak attack. The best I can think of is a Reef Shark. Then they would be immobile and flopping on the ground. I am worried though that any non-polymorphed bad guys will just drain the temp hit points to end the spell on them.

Any ideas on a beast with decent hit points, weak attacks and preferably no walking or flying speed?

 

[humble minion]

Giant Snail, from Witchlight? Only 22hp though, same as a reef shark, so might not be enough for your purposes.

 

[MarkB]

Killer whale? Nasty attack, but only 5 ft walking speed and 90 hp.

Alternatively, throw in a bag of rats and cast it on them to turn them into T-Rexes, then wait while they fight it out with the bad guys and go in afterwards.