Abandon Ship

You have to defend the EMP and Mario from Di Ravello's onslaught. There are multiple emplacements in the area like machine guns and flak cannons so use them to your advantage. You'll have to deal with multiple fronts and you'll have to be actively changing positions and manning the emplacements. Keep destroying enemy soldiers and vehicles until Dimah gives you the prompt to activate the EMP. Use it to destroy every enemy vehicle within range.

Making a stand

In the second part of the mission, you have to deal with more reinforcements, including gunboats, gunships, and helicopters. There will be a brief respite between these major waves so make sure to restock your ammo and get new weapons from marked weapon racks in the area. Fortunately, you don't have to deal with any tanks but the enemy corvettes are not something to be trifled with. Keep fighting them off until you get another prompt from Dimah to activate the EMP the second time around.