Meganium @ Leftovers
Ability: Thick Fat
EVs: 252 HP / 252 Def / 4 SpA OR 252 HP / 4 SpA / 252 SpD
Bold Nature
- Giga Drain
- Earth Power
- Synthesis
- Roar
The ability Thick Fat negates Meganium's common weaknesses to Ice and Fire type moves, leaving it with only 3 weaknesses in Flying, Bug, and Poison (which aren't the most common types). Earth Power adds to it's coverage options (there aren't many). Roar allows it to be a (more reliable/better) phazer (it has dragon tail but that can't break subs and is only 90% accurate).
Earth Power makes sense considering Meganium is basically a giant plant (plants grow in the earth, in case you didn't know), and considering it already gets Earthquake. Roar because it looks like a Dinosaur I guess, and it has a big mouth so it'd make sense for it to roar. Thick Fat because it's a large, fat 'mon.