This is an important point Naxyr. I myself main WHM but also have a PLD with decent gear. I helps when you get to see both sides.

Regen and Medica II are great healing spells, but the first thing you need to learn about them is that they should never be on when new mobs are about to enter the fight. So this applies both to a pull and anytime in a fight when adds will spawn.

For example the split on coil 1, Garuda or Titan Ex last phase. WHMs must time their heals so Medica II is never up when adds spawn, and preferably not any regens either.

And of course healing in general should be as "little as possible" when things join the fight. Stoneskin before pull gives you a few seconds at start where you dont need to heal. But in fights where things spawn you need to work hard to top tank (and possibly raid) up just before they spawn.

This is basically a matter of healer skill and experience.