I don't remember doing any extra training at the end of FFXIII. However in FFXIII-2 it's pretty much a requirement as the boss is overpowered beyond belief.
I think I was doing poor because of my team set up. I had Lightning, Sahz, and I think Snow. I refused to change the team since I didn't like the other characters, (except Fang, she's fine). I kept losing, and I couldn't stand it anymore, so I quit.
I do like Sahz but he's the weakest character (in terms of damage) of the bunch. Snow is alright if used well but I never needed a Sentinel, so I barely used him.
AFAIR it's really important to have a Saboteur and a Synergist in the party.
With Hope, Vanille and Lightning it's terribly easy. All 3 are Medics, all 3 are Ravagers, Lightning is a Commando, Hope is a Synergist, Vanille a Saboteur.
I completed FFXIII without dying once on my second playthrough
I was using Vanille, Fang and I don't remember the last one
my 6 strategies were
Fang and Vanille are the best Syn and Sab
Start with Syn and wait until everyone has Haste and all the other buffs
Then switch to Sab and debuff all the ennemies
Then use Com and Rev to stagger the ennemies
Once the ennemie is staggered use rav to bring the stagger percentage higher and a few seconds before it ends switch to com for max dmg
If the ennemie uses a powerfull atk put everyone on Sen and then heal with everyone on med