Move your invader (Beach Boys) unit to the Depot in the SW to get the units from there to the mainland and move your battle tanks from the depot in the SE forward. Move your air units closer to the action. Move the scattered units towards your HQ, lone units are easy prey.

Important are combat positions. Adjacent units can grant attack or defense bonuses and if a unit is attacked from opposite directions defense values are reduced drastically, so make sure you are the one surrounding.
In round 2 or 3 the enemy bombers will attack, but they shouldn't be able to do too much damage if they can't surround you but you can use your mosquitos from the back to surround and damage them heavily or even wipe them out in the next attack. Wiped out units can't surround you and mosquitos are helpless when attacked by ground units. When the bombers are gone the level is almost won.

You can watch the computer before choosing your turn. If you see which of units will be attacked, damaged or destroyed you can plan your moves better.
When it's your turn attacking you can enable the end turn symbol to make the computer think faster and when it starts moving cancel the symbol so the computer moves more slowly so you can watch it better in difficult battles.