First, make sure to maximize the level of your castle. By upgrading your castle, you will get a bonus on experience every battle, up to a 100% bonus.
You may also have a ressource you are not using that much while it could help you a lot on your clerics progression: transparent shards and crystals:
When you use a shard on a character which level is lower or equal to 19, he will win 1 experience point. Crystals are working the same way, but on a character with a level superior or equal to 20. This can help you to get some quick levels on your clerics without even fighting. As these are kinda rare and long to collect though, make sure to use them only on the characters you want to maximize first.
For the battle phase itself, I usually try to find maps with bottlenecks part, such as maps with bridges:
On those maps, I usually place a character which can't attack back an opponent in close combat, such as a mage or a bowman, and my healer behind. The character in front has to be resistant enough to not die, but still should be able to take damages from an opponent. I then wait for an opponent character to come attack this character, and I heal this same character for several turns. As you may have noticed, the experience won from healing decreases with each turn, but it will still allow you to gain a fair amount of experience, especially for the first levels. When the healing doesn't bring you any more experience, just attack the opponent until he dies, awarding you with a fair amount of experience.
When I am not on such a convenient map, I always try to adopt this strategy: one character protect my cleric in front and damage the opponent character without killing him, and my cleric then finish him to get the kill experience.
I don't think there is a specific faster way to evolve your clerics than to play on all those parameters. To sum up:
- Level up your castle to the Royal Castle to get a 100% bonus on experience.
- Use transparent shards and crystals to get some free levels without even fighting.
- Take advantage of maps with bottlenecks to get some free experience on healing and killing opponents.
- Rest of the time, protect your cleric, damage your opponents with your other characters without killing them, and let your cleric finish the job.