While I can't give any specific advise, remember that medica II (and whispering dawn from the SCH) give group regen. I see a lot of WHM's using medica II followed by a medica when there is actually enough time to let the medica II regen completely heal the whole group. Keep in mind that Whispering dawn is also a heavy group regen (assuming it's combined with rouse). Also medica II isn't always the only choice, medica I sometimes is more than enough. As WHM you'll have to learn to when NOT to heal. Preserving MP is harder than on SCH. Minimize overhealing, know when you can let your group regen tick and when not. Especially in SCoB there are alot of situations where you can let the regen just tick.
(Also remember that you now have to buff the whole party with protect and Stoneskin :P Congratulations for this tedious part of the job)