Beat the game, as in, beat the final boss? You will need some more levelling up, I daresay. Mt Battle is your friend, and with the 'winnings' there you can get some useful TM's from Mt Battle as well (e.g. Suicune with Ice beam FTW). I don't quite remember, but having the majority of your team around the... mid-50's may do it? Upon checking, the second-last battle (and the few before it) have Pokes around level 55, and the last has a bit of a jump in it with Pokes around level 60. A hint as well - th first two pokes are Slowking and Slaking - and Slowking likes to use Skill Swap with Slaking which isn't good, so try to get Slaking out of the way.
Plus, the Rain Dance tactic will help you greatly - Raikou use Rain Dance, Suicune use Surf, then Surf and Thunder away. Quagsire would also then benefit, but Entei wouldn't.
I wouldn't shy away from levelling up Umbreon and Espeon as well - they are a good wall and attacker respectively.
For the snagging made easier, you can use the Master Ball glitch as well (if unsure on that, just PM me or a quick good search will tell you it), and if you want to know what to do to get to the end, you'll have to be more specific, and there's walkthroughs also via a Google search (IGN has one, methinks).
Hope I helped.
