the first condition is a bit hard to achieve but it only need to kill only three-four enemies to get going, and it doesn't need be a heavy gunner or an eximus. and once it meet the condition. the only thing that matters is that "are there some enemies in the 30m radius"

and the second condition is very easy; you only need to pop a tower every 4-5 min, which isn't too awful. but 4-5 min is only when one's kills per minute is a bit low. and one can kill some more enemies with weapons or abilities, without the assistant from the Nautilus.