In the 8th level, where you have to blow up the aliens' entrance points you can farm some exp. Place at each entrance a soldier with the assault rifle (the 3rd one with the bayonet), in defense mode. The aliens will get killed instantly without using ammo. I had 3 soldier with the assault rifle, 1 sniper and the armored marine (these other can be used to clear the level of the other monsters and, in my case blow up the 1st entrance). You can level up a couple of times before you run out of aliens (around the 25th turn).
The demon combo disease/cripple/curse is lethal for the bosses. Remember to cast silence and curse whenever they wore off. Spend your skill points mainly to gain this skills plus on mana and critical since this will affect the efficiency of the spell death.
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