Using your healing spells should recover some HP too. I beat the Red Gigas on my 2nd try I believe. Just try to stay alive.
You concentrate fire by making sure that all your rounds hit at the same time. I don't remember what weaponry you have at that stage of the game, but here's an example.
Fire a torpedo, and set it to hit 2 turns after the time you fired.
Fire your sub-cannon in the next turn, and set it to continue into the next turn.
Fire your main cannons in the next turn when the other two shots are scheduled to hit.
In the other turn, heal your ship, or use focus or something. If you concentrate all your fire on the Red Gigas right before he shoots, he will miss you completely.