The excedent (and useless) exp in each unit should be used to increase passively some atributes like HP, damage, spawning rate, population or damage reduction.
I love the concept, the weapons are wonderful, but I found the campaign (the free campaign) too short. I think that's not necessary lock the Reversed Survival here. In fact, a custom Reversed Survival when we could choose the gadgets, weapons and arena should be great. 4/5, and I cannot wait for the sequel of The Final Death Wish.
This is the "sequel".