Nothing really can make Leafeon great, but you might consider Roserade UL and Rainbow Energy (Poisoned and Confused when it's attached). Skuntank G works as well; poisons both actives, but you can always retreat or warp into another Leafeon. Magmorter SV is good as well; any time you evolve a Magmar into it, it burns and confuses your opponent's active.
Unown RETURN (HGSS 54) is well paired with Roserade to get energy on it back to your hand. With Magmortar, you would have to run Seeker and MAYBE SSU (although I wouldn't).
All in all, though, this deck doesn't work really really well. You can get OHKOs often enough, but only with a pretty good opportunity (of course, those special conditions can work wonders, too). Just a word of caution.