Eunomiac, sorry it took me awhile to see your post. Darthwesix has already spilled the beans on my favorite flight technique, as it's easy and completely maintenance free.
The Flight spell in AM2 is also easy enough. You do have to either luck out on dungeon loot (infinity orbs find their way in there sometimes) or wait until magic level 41 before you can nab it. I wouldn't bother too much with the Hourglass ritual myself. It's easy enough to recast. And later on you'll get the duration modifier which helps a lot.
Thaumostatic Harness is the one that's potentially available the soonest, since it doesn't require any exotic ingredients. The Potentia is renewable by burning charcoal, and a full tank lasts long enough to be worthwhile. You'll need some inventory spaces for refills, but with the backpacks and ender bags and so on, that's unlikely to be a big issue.
Agreed about the spire. Not worth the investment unless you have some kind of automated Aer farm.
The Gaia Guardian, in its first iteration, isn't TOO bad a fight, especially if you have a Talisman of Remedium. You won't want to wear your AM2 armor for it though. It does a lot of armor damage. Unbreaking diamond armor is better. Terrasteel better still. You may need to adjust tactics to compensate for reduced Ars Magica potency. You can't chain hits on the Guardian; it teleports each time it takes damage. Focus on attacks that hit hard, but slowly. Keep a faster attack technique on hand though, since as it gets low on HP it'll start summoning swarms of normal mobs to help out. Also, resistance to knockback via the tectonic belt or an artifact piece is -highly- recommended. Oh, also, you can't fly during that fight by any means. So don't bother bringing your flight bauble.
The tiara is a great flight technique though! Almost as good as the belt of flight, and leaves the belt slot open for other baubles. Just be sure to have a good supply of mana. The ring slots are useful for upgraded rings of aura and mana, for this purpose. Later you can make a mana mirror to connect to a dedicated pool.
Don't bother with Elementium in my view. It's only slightly better than iron armor in defense, and the pixies aren't all that potent. Maybe if you use the ring and sword from the set...but there's MUCH better weapons and baubles to be using.
I've been using a suit of battlemage armor to level it, but I also have a 'high threat' suit of diamond artifact armor. Regeneration, damage resistance, knockback resistance and many other abilities make it a nice suit to wear in situations where I'm not sure if the mage gear will be enough. Great for dungeon crawls. Unlike vanilla mob armor, artifacts have no Thaumcraft aspects, so are essentially useless except for vanity displays if you don't use them. A lot of them are junk, but you can find some real gems out there too, if you keep looking.
The Runic Armor is not SUPER great, no. Big bad mobs will swat it fast. What it IS good for is protecting your health from hunger-sapping hits from peons. In a dungeon environment, it's not uncommon to get swarmed by lesser mobs. You may not take much damage, but any damage softens you up, and forces you to eat and deplete resources. Runic armor will block several hits from vanilla mobs, and recharge quickly and at only a small cost of a resource that isn't terribly useful for much else in combat (your wand foci will be playing second fiddle to your AM2 spells until they're super upgraded, and even then will often be overshadowed). And even blocking one hit from an infernal or boss mob can save your hide.
I am using an Amulet of Runic Armor most of the time. I have a Talisman of Remedium for harder fights, and fights I know I'll need it. You may be able to stick Repair on it with a book, but I just put it in the Ars Magica Arcane Reconstructor. Expensive in etherium, but I don't use it for a whole lot else.