To me, it seemed like challenge mode required much less thinking since you dont have to worry about what programs to use each stage. However, I can see that they are designed to let you find a way through somewhat less intuitive alternative for each stage, which is very interesting indeed. Easily best tower defense game Ive played recently... this game lets you actually think in depths.
Inferno mission: As some others have pointed out, only things that can stop Inferno from advancing are 'towers'. Equip as many ranged units as you can, and equip one unit for building towers. Spam towers while ranged units hit boss safely. If you do well, you will have killed boss just before it reaches your hero.
Colossus: Equip 1 little hobit or goblin, and equip ranged units for all other slots. The idea is that since colossus can literally 1-hit kill every unit, its best to let him waste his attacks on few fast small units(which respawn fairly quickly) to hold him, and using ranged units to hit him safely. Using Rage spell on ranged units is very good, but it is not necessary.
I can go up to 10,000,000 hits...
Actually everyone can go up to infinity (if one has infinite time and super computer)
by just covering entire floor with launchers...
Lots of valve upgrade and buzzsaw, lasers and cannons will increase the speed.
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