To get XP for a monster kill, you just have to be on the monster’s aggro table when it dies. If you are in a party, you get credit for the aggro of everyone in your party as long as you are on there with at least 1 aggro point.
As a healer, you generate aggro on monsters when you heal someone that they are attacking.
So basically if you heal someone that the monster is attacking you should get credit. And if you are in a party with that person you will get full credit for what they do as well.