Build up your bonus lives by taking as few hits from enemy tanks as possible.
Find a good position to take enemy tanks out from. Do not stand still, keep your tank mobile within that good position. Abandon a position if overwhelmed. Move forward when firing and then reverse away after firing when appropriate.
Keep constant fire to keep the enemy off balance. When you fire, turn your tank slightly while letting a volley 5 shots to get a wide range and maximum chance of a hit. The computer tanks can't as easily avoid a wide volley of shots.
Use angle shots to your advantage. Try to trap enemy tanks in bad positions where shots will richochet all about them and take many hits.