Training tips?

Well I finally made it to the final fight with Hein...and I lasted 'till he brought out horntail and his dark fury (Drena hadn't been killed though...). Next thing I knew, I was getting bombarded with omega dragon crushers. Died pretty quick. I figured my party isn't up to snuff yet (level 75 Antlan, Serene, and Zeru with level 60 Lenna). SOOOOOO where would be a good place to train? Other dimension isn't much of an option with warp stone not working in hades maple world. So any tips from the pros?

it doesnt sound like your underleveled to me o-o
I think the battle with hein is a bit more about strategy and such
cuz I could almost take down his final form with level 62's
if you want to train, the efficient but somewhat slow way is priests in lith and other places
or zakum if you want a faster way to do it since your lvl 70

You could grind on priests, as myfantasy noted in Lith, or the Oracles on the floor before Hein. If you didn't play FSI, you already lost the best chance for training: The DO Rematch Crystal. Zakum is out of the question, because you need access to The Other Dimension, which you don't have.
I really don't have much to say, since I killed them with a level 99 original party, but I can offer some pointers:
Make sure your healers have good attack/buff skills: Aegis for Zeru, Magic/Mystic walls for Serene, Party Heal 3 for Serene....Yadda yadda...
Never gang up on Drena. Kill the other revived DO members first, because even though they look like they're in a Dark Fury, they're significantly weaker. (At least, Maru was.) If anything, if you summon Hein's Dark Fury, KILL IT FIRST AND ASAP. As long as you leave the other members alone, they don't pose much of a threat. Horntail isn't either, as his Aegis barrier only seems to affect himself and the Spirit of Agony. Also, the alternate DO members seemed to have two Dark Fury forms, so it could be like the Ellinia Battle + Hein. Also, don't bother killing the spirit of agony with Horntail out. That thing is cheaaaaaap....
If you played FSI and carried over Tuna Soups, USE THEM!! Fried Furious Fishes? Fried Mad Fishes! Use whatever! Zeru the Coder (Zeru's obviously a schizophrenic) told me that Transformations give base stats, so if you need a boost of power, use them! (I ain't kidding. Zeru: Bzk Vow+Psycho Sword=60k, Bzk Vow+Fire Trans+Psycho Sword=74k. How do I get Psycho and Trans within two turns? It's called Fried Furious Fishes, Tysvm.) You can Fish after the final battle, so don't save them, dangit! Also make sure you have PARTY BARRIER OF DARKNESS. Hein likes to spam Possession, so that's always a great skill to carry.
If you're REALLY out of options, get out of there and go back to the Base, repot, and TRAIN TRAIN TRAIN TRAIN TRAIN!! Go to the single living chicken in Henesys and make some Scrambled Eggs (Provided you have Cooking set+oil+salt)
Anyways, good luck. Hein's a hard boss, but he isn't invincible.

Hot diggity. Thanks for the tips you guys! I realized that I was going in with only 10 mp elixers haha. I'm gonna' try again later today and take neo's advice! Thanks again!

Oh yah, one last thing. Snoop around Hein's Castle Floor 3 for party barrier of darkness. It's somewhere near the left side. Also, you're welcome. :D