You can beat lvl 18 with Angels/Titans...no melee units. How? Let the ones getting beat get beat; don't replenish them and just keep using angels and titans. He won't attack at all if there are no melee fighters.
Very nice bug: since my Firefox hates flash, I keep having to restart while I'm in the middle of buy/equip for survival mode. Every time I start back up, my inventory & money doubles. Nice!
I had a hard time stopping my play. This very addictive game left me wanting bonus, darn near impossible levels at the end. Oh well, time for more title squishes.