Mantine... How life is unfair for you! You were designed as a counterpart to the ever useful skarmory but yet you fail in taking off like the the latter did. Your main issue is the fact that you lack recovery moves and you have to rely on rest, or water absorb to heal you up! While skarmory can set up spikes, stealth rock, phaze and attack while tanking major physical hits like a champion due to it's massive amount of resistances. You, on the other hand, are not that useful. Your typing leaves you with only two weaknesses, but, unfortunately, you have the 2 worst weaknesses a tank could ever have. A 4x weakness to electric can OHKO you even through you mega Sp DEF stat (no tank ever wants a 4x weakness for any type). A second weakness to rock types is what seals your position in NU reducing a major chunk of your bulk (no tank wants a weakness to stealth rocks). Rain dance is your main asset! You can effectively set up rain on a timed switch in, but even this job hazardous as it makes opponent's thunder perfectly accurate! Maybe you can use aqua ring the way skyla did to defend swanna from pikachu's thunderbolt! Might work!
All is not lost, 140 Sp DEF is still amazing and any special move that is not electric type will do small damage to you. Toxic is a nice option as you can take out something that you don't like in your opponent's team. Toxic, usually means that this pokemon won't last long on the battlefield. Scald is another nifty toy that have the potential to weaken opponent counter that tend to explore your softer physical side! Confusion and swagger work too... But, as you can see, Matine relies too much on hax to be effective to the point that most of the time it will fall in battle easily!
Sadly, mantine is not a dangerous pokemon, skarmory is a million time more useful, but, it can be a very difficult thorn in your opponent's back to remove in some situations. The only thing that mantine has that can make skarmory jealous of is a pre-evolution mantyke. (Although I'm not sure if that's a good thing).
My idea of a set
A mantine quarantine!
Mantine@leftovers / Damp rock
Ability: Water absorb / swift swim
Nature: +Sp DEF / -ATT
EVs: 252 HP / 252 Sp DEF / 4 Sp ATT
Moveset:
-Scald
-Toxic
-Rain dance
-Protect
Simple, a typical stall, wall, tank, you name it, set. Set up rain dance, toxic any pokemon, stall out toxic damage with protect and scald to attack while having a chance to burn a physical attacker! The ability and item are dependable. If you decide to run swift swim, give mantine leftovers so you can heal a little after each turn. If you gave mantine water absorb, damp rock is better as it makes rain last longer. The healing can be done with a predicted switch in to a water type attack!
Mantyke with eviolite is a pretty good wall in LC... It can run a similar set as the one above with some effect!
BTW...
Can you say "I'm flying a mantyke kyte on a bike" several times rapidly? 