New PSP owner and I've just beaten this game including Metatron at level 71 Really a nice and simple RPG, recommended.

Its not necessary to be level 80+ to be able to beat these two ... If you're properly equipped and know what you're doing, I believe you can even beat it at level 60.

I'd recommend you to use Meladee and Sogreth ( Tatsuya or the main protagonist is mandatory ) ... because Sogreth's defense ignoring skill, Nutcracker is a very useful skill when you meet those with high defense, and most boss monsters have it, not to mention his tremendous tank-ability that makes him the toughest out of all in the game. Only one slot left now and you'd definitely need a healer. Meladee is your best bet because she has all kind of heals and resurrection that brings back dead teammate to life with and heal him/her fully.

Yuno has a reviving skill but that won't fully heal your life which is very essential, specifically in the fights against Sheng and Metatron. She doesn't have Zed heal ( team healing skill ) which will be a pain in the ass. Her damage output is far inferior when compared to Meladee. After all those marvelous healing/reviving skills, Meladee's damage output is on par with Tatsuya and Sogreth, her last skill hits random target for ten hits, which is at the same time a good sweeping and bossing skill.

Some strategy for Sheng and Metatron :

Firstly, you'd need to get yourself equipped with the HP restoring accessories, which are the best you can get from the Four Guardians and Maree after retrieving the all the items in Earthrift. Get yourself equipped with the last weapon ( Firewyrm Blades/Axe of the Snow ) good accessories which I'd recommend Breastkin Crest or Firewyrm Crest. Breastkin Crest is much easier to craft and its as good as Firewyrm Crest.

Now, when the battle starts, use Tatsuya to summon Vyne immediately. Cast Lifetaker which is a unity skill of Tatsuya and Meladee, this skill will last you through the round with full life as it has pretty sick damage with a life absorbing abilities so you can end the round with full life ( Sheng/Metatron will probably attack you first as they are very fast ), cast Sand of the Goddess with Sogreth since you won't need to spend BP when Vyne's around. The next round, cast Super Traveler of Tatsuya immediately. This skill is so awesome that it boosts the unity skills together, which means you can cast a unity skill with either Meladee/Sogreth and it will double the damage, as long as Tatsuya is one of the unity member. With this tactic, you should be dealing 400 and 600 respectively, with the use of Lifetaker and Sands of the Goddess. I will recommend you to use Lifetaker even though its damage is a tad lower because it will keep you from worrying Tatsuya and Meladee. Rinse and repeat and the boss should be dead pretty soon.

Metatron is almost the same except the fact that he has a longer life and you must lock when he loads his spell cannon which will kill you in the next round if you don't.