While this is an old question now it is more relevant than ever after the buffs Mantine got in Pokemon Sun & Moon (+20 base HP, +ROOST).
So mine is:
Mantine @ Leftovers
Ability: Swift Swim
EVs: 200 HP / 200 Def / 98 SpA / 12 Spe
- Rain Dance
- Air Slash / Toxic
The above mentioned EV spread makes Mantine a decent offensive threat while maintaining its purpose as a wall. Bold Nature invests in Mantine's Defense which is a bit lower than ideal.
Roost + Scald + Leftovers are mandatory to support better Mantine's walling capabilities.
Rain Dance doubles your Speed (who doesn't want to be faster?) AND your firepower making Scald a considerable threat with 30% burn chance.
(Also it can help you switch in a special Electric Sweeper and spam perfect accuracy STAB Thunders!)
Finally Air Slash is a special STAB Flying-type move with good 75 Base power and a high 30% chance to make your opponents flinch. It serves Mantine on the offensive side.
Alternatively, Toxic makes Mantine a better wall that can cripple your opponents with bad poison.
Also Defog should be considered to remove entry hazards (But it's a gen 4 only move).
And Haze (An egg move) can be used as well to stop set-up sweepers.