One of many great things about this game is that doing what you describe can actually be a viable strategy, if combined with certain characters (ie, Sheba) or equipment (ie, loincloth).
Though really, the game's easy enough that you can use whoever you want and get by. Just be sure to use pouches wisely and remember to stay at inns.
Chain Attacks are your best friend here.
Many enemies will start doing ultra cheap/deadly attacks when their health bar hits a certain point, and otherwise get generally powered up. The best way to avoid this is just to one shot them before this happens. If you're putting seals on him you're on the right track, but remember that multiple seals are useless unless you chain attack him. Ideally you should start the attack BEFORE he enters his power up state, but it's understandable you might not have enough orbs to kill him before then.
Try padding your party with blades with LOW attack power. Ie, generics that haven't had ANY powerups applied and are still in their base form. That way, you can stack elements without getting the enemy's health down too low. Use pouch items that make Special and Party meters charge faster to make the job easier.
When he's about to hit 50%, regardless of how many orbs you've got, start the Chain Attack. You may not get Full Burst, but you'll probably be able to kill him.