I completed this while going for "The Untouchables" and "Exquisite Timing".
On the second to last level, you are only allowed to bring THREE soldiers to a battle. I picked 3 Grenadiers, and had finished building out 3 EXO suits by the time I got to this mission, giving each soldier a nice long range rocket attack. I used Save/Reload to try and get better rolls (and trying different approaches), but mainlyvjust took it slow. There is no timer on this level! I dropped grenades and rockets when the creatures were well grouped.
In the end, I had to fight 4 groups of enemies, for a total of 9 creatures.
Hope this helps!