what are we looking at for a decent gear set?
Ice dmg+ magian staff, thunder dmg+ too if you feel like it, and ideal atmas (Beyond, Minikin, Ultimate) are the most important things you can get in the damage department. Anything else you can get is just icing, really.
Most important thing is to stack as much PDT as you can (up to the cap, which I'm assuming is "51%"). Earth Staff is a no-brainer, Jelly Ring as well unless you've already managed to farm yourself some superior Dark Rings in Konschtat[A]. Cheviot/Umbra Cape is great, Goliard legs or A Shantotto Ascension legs, Twilight Torque.
Bear in mind though that whenever you are not nuking, you will want your PDT gear on. This *especially* includes when you are casting Sleepga. You don't even want to take off your Earth Staff for it.
just run through the -ga3s?
You'll want to Sleepga the mobs. Even if they are MNK types and your PDT set is good enough to take 0 damage from them, you don't want to be taking hits while you're casting your nuke or else you won't be able to (safely) swap out your staff for the nuke.
should I be casting ice spikes as well?
No, mobs have a habit of hitting you right as they're being slept, and waking themselves up with the spike damage (you'd think this kind of thing shouldn't be able to happen, but it can)
You'll also want to keep any Igqira/Genie gear off, because you don't really want some mobs getting cursed and falling behind the rest of the train.
If Mana Wall is down and you need to get an uninterrupted spell off right NOW, yes. Manafont is more for the uninterrupted casts than the MP in this situation.
If you have maxed pearl+azure AND there currently aren't a bunch of people in your Abyssea zone leaving chests on the ground and screwing up chest drops, you will usually get at least one restore chest with each batch of mobs you kill, so you can be a bit more liberal in your JA usage.