This kind of question is somewhat open-ended. The "best route to go" to 99 depends on what your objectives are. You could just leech, although I don't recommend that. You could join any aby party, and kill things one at a time with your biggest nukes, and ride temps as people key them - you would at least be contributing.
But what I usually do, for myself (mule, since my blm is long-since 99) or my friends, is to just go in and wreck things while others key chests for me. My specific strategy is to use a camp with magic-aggro mobs (dolls, detectors, snowballs, golems, etc), pop manawall -> manafont -> ISL, run into the middle of a group of mobs, and cast my biggest -ja and -ga nukes until they all fall down. -ja spells are just bigger versions of -ga spells that also increase the damage of subsequent casts of the same nukes. So depending on how much HP the mobs have, my cast sequence usually starts with Blizzaja -> Blizzaga III, followed by whatever else I think I need to kill what's left. When my ice timers are down, I use thunder series nukes. Depending on how many mobs, a convert may be necessary midway through. Then I find another ISL chest and start all over. Depending on how much help you have, you can vary this strategy to include someone else pulling/gathering. You can also pop manafont up front, -ga everything down, then blind-key your blue chests until you hit an ISL.
Merits are pretty much the same as they have been: ice/thunder potency, unlock each AM2, INT, elemental/enfeebling/enhancing magic, etc.