First of all, what exactly does Renew do, and where do you get it and Hastega?
Second, where/how do you find Ultima?

Renew fully restores HP of all characters. It's pretty useless IMO since Curaja + Faith/Mystic Armour is usually enough to fully heal your characters, except at very high levels and/or using Bubble.

It can be bought in the shop in either the Westersand or the Estersand village. Can't remember which. Not sure about Hastega.

I read you can find Ultima in the Great Crystal, but I haven't actually got it myself. I don't know where exactly.

Hope that is of some use, no doubt someone else will be able to tell you where to get Hastega, or you can look it up on GameFAQs.

Ultima would be in the Great Crystal. It's quite a hassle to get there and I can't explain it to you just like that. You're best off searching for a map.

Also about Renew. I found this spell to be rather useless. Curaja is all you need, even at high levels without Faith and with Bubble. It heals more then enough and costs less mana.

Sort of like FFVII's Full-Cure --- way too much MP for something you don't really need.

I know, I'm just a freak about getting all the spells and such. I thought it'd be something like "cure" and "esuna" put together...so much for wishful thinking, ne? About Ultima - all the maps seem to be wrong, for whatever reason. The best map I've found seems to be backwards.... I guess there's really no way to tell, there's probably a walkthrough on youtube or something.....

Finding Ultima was a bit of a pain in the ass, but I found her eventually. I gave up on maps and relied on step-by-step instructions from one of the FAQs at GameFAQs. I forget which one, though, though offhand it may have been one of the esper-specific guides. Even with that, I think I still got lost at one point, but managed to work it out anyway.

There's only one reason for getting Renew, that's the Sky Pirates Den.

The renew spell can be useful against high lvl bosses during side quests. I remember using it alot to beat the Hell Wyrm, since it's special attack took alot of damage, I would need to be at full hp to survive it. Since I was lvl 70'ish with bubble belts, Curaja didn't bring me to full hp.

Renew works really well for killing abstyeels, doesn't it? Maybe?

I seem to recall that trying to use spells / items like that as instant-death against undead creatures doesn't work in this game. Another reason why I love this game's undead.

Pheonix Downs still work.

Or are you referring to something else?

Not that I know of. I thought that Phoenix Downs didn't work against most undead in this game. Or maybe it's just the rare / boss undead...

Phoenix Downs do work, even on some rare game. It's just that the undead have varying levels of resistance to one-hit KO healing items/abilities. And bosses and some rare game are completely immune.

Mmm, so in other words, it can work, but it's a crapshoot.

Yes, which is why the Pheonix Down works best when you're equipped with the Nihopalaca.