Forget the stinger and sniper, all you need are gunner and flame unit fully upgraded plus mortar unit. Tesla is OK for some of the later stages, but does minimal damage even when fully upgraded.. Stinger fires too slow and sniper is too hard to use - only covers one lane so you have to move it around too much and be very accurate where you place it. Use the mortars to hit the bugs as they come towards you and the gunners/flames to mop 'em up as they get closer. Replay the levels once completed to build up medipaks, stimpaks and airstrikes - you'll need them all for the final missions.
Alright! Zig Zag - don't use/buy red element at all. Only upgrade interest multiplier to 8 - save rest to reset elements. With careful cash/point management you can have enough money to build 21 snipers +5/6 maxed out small blue towers = enough to destroy last wave!
Cool TD game, absolutely love it. Can the armour plated dudes with 2 million HP in the last wave of Zig Zag be stopped with anything? I've tried fire/snipers and slowing em down and using the green and purple towers on em and haven't got close to killing one! Any hints? Please?