Rzarektor is right, there are places on the map which offer good shielding against incoming attacks even at quite high attack angles. Predator and jSimpson are probably the best tank players I've seen on RC1 at utilizing this strategy as a defense vs AH. Also they are always tucked between buildings or on the opposite side of buildings to enemy AH etc which makes them a lot harder to find in the first place.
As these guys have suggested you should use the cannister or HMG as your secondary as it can be quite effective vs helis. If your playing in a game when the AH is not really focusing on armor you can switch and use guided shells. Kinda gotta adapt your strat a bit depending on who you are playing against.