Me, I waited until they had gotten to 20 as healers and promoted them to Sage/Valkyrie. Being a first-tier healer grants lots of EXP, so their Magic stat will be high enough for them to create havoc with a Wind tome.
Grind for Galeforce later.
Confession Time! jRPGs are pretty much the best thing that ever happened to Video Games - Soanevalcke6
Maribelle makes a pretty good reclassed Mage, so if you have an excess of second seals that's not a bad idea.
Otherwise, you can level clerics/troubadours pretty quickly by simply healing every turn and not ending the stage until you've healed everyone... Although this is assuming you only have one deployed.
For that reason, I would level Lissa to a level 20 cleric anyways. If you aren't grinding, neither Maribelle or Lissa are going to be able to tank. The amount of experience they are going to get healing at low levels is likely to not be much different then the amount of experience they are going to get killing things on the player phase against promoted units.
Official claimant of Mem Aleph for the theoretic cause of promoting Shin Megami Tensei IV of the 3DS.
If you are going to level them to 20 as healers, I`d rather level them to 10 in two base classes (Cleric - Troubadour for Lissa, Troubadour - Mage for Maribelle). But then again, I`m a completionist so I tend to grind for all skills anyway...