Heal Anytime/Anywhere*
*This only works if you have two devices.
Step one: get the C-Gear from Bianca so you can trade your Pokemon from anywhere. Next, if you have two DS consoles and two copies of the game, you can trade it back and forth between devices. When you get your traded Pokemon back, the life meter will be full.

How to Beat Clay
Okay, so Clay is one tough contender. I've seen it done with a Grass, Fire, Water, Rock and Psychic/Fire Pokemon, but all of them were over level 30 (most over level 35). So the first tip is to level up wherever possible.
Next work on some high damage moves, the kind that can deliver 65+ attack damage. Try to make sure your Pokemon use attacks that are the same type as themselves too, and this will increase the damage by 50%.
Finally: items. If you can get hold of the Dire Hit, you'll amp the Critical Hit ratio of the Pokemon once. X Attack increases the attack stats of the battling Pokemon while X Defend does the same for the Pokemon's defence.