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 has:
Ability: Swift Swim
EV's: 200 HP, 200 Def, 98 SpA, 12 Speed
2. Rain Dance
4. 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 badly poisoning them.
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 swords dancing sweepers.