The only two tatics I have found to work well consistantly a the following;
1) For Imperial: Walk slowly and steadily into enemy bases with an AT-At clearing all buildings with its chin gun, supported by a small number of anti infantry and anti air vehicles. N.B. Always always remember however clear you think the base is its still better to have a chin gun shot in reserve before you advance.
2) For Rebels defending against repeated large scale attacks: Keep two Infiltrators in a hover transport in a flanking position shortcut key it (shift+1, worked well for me),
Once the supporting troops are thined out a little click <1> move in grab the infiltrators and capture the AT-AT then see 1.