I think that is is a really good idea. Especially in the last few boss fights, you will need the extra mana to recover from the losses.
As for the driller, I did it with 4 units. The idea was to utilize his weaknesses (as well as the weaknesses of his summonings - -50% mahgic resist) to the best of your ability. 5 units is simply begging for constant gunshots which means a lot of damage you can barely fix in time.
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