Not too bad
At first I found this game practically impossible to finish until I discovered a simple stategy. Repairs are way too high, especially for the big guns, so the idea is to never have to repair them. So I made a big pile of expensive guns, and then just kept spamming a pair of m60's spaced a bit apart between my big guns and the entrance. The enemy only targeted the m60s, while my ever growing pile of big guns blasted everything.