It's such a skill based fight It's very difficult to advise on units required. It goes anywhere from one sho to 500+ units.
I usually think of the grandmaster fight as fun because of like his sp 1 and those challenges that you have to complete it is not. like a boss fight that is hard, it is more of a fun 1. so it can go as far as 1 shot or 500 units as @IKON says
Is Magneto immune to reverse controls from GM as well?
Zero research maybe 800 unitsWatch a couple videos 400-500units2nd attempt 300 units, there's really no substitute for experience with this fight.
If you aren't prepared for reverse control then 800+ otherwise less than 800 units,(it's my rough assumption by seeing your rosters & progress)If you can take that life cycle (to block his sp1) path then you can save many units because most of the kills comes from GM's sp3 (5 blunder tokens). struck by unblockable sp1 will give you 3 of theseYou can also save many resources because you have aegon one of the best option for him, crits or big damage in GM's wounded state (~6 second window) is the key, bwcv is also good