Well you are facing a strong underwater only army of probably atleast 6000+ gold so will probably need atleast a similar costed force to tackle it.
Definitely dont use deep ones due to their abysmal MR. Vast hordes of atlantian spearmen and war shamblers backed by antimagic would probably be your best bet. Other low resource indies could also fill out your ranks.
One possible(but untested) way would be a MR focused basalt king out in front to soak up mind blast hits(they should target him due to high health/large side/closeness)and maybe do some damage too - Just let the king buff with earthpower, iron will and X skin and fireshield(also phoenix pyre if you have it).
You can(hopefully) see an example of this below or in the attachment.
edit: OK, im pretty sure i messed that up, can people see the picture?
