Well let's see here... Both Mawile and Volbeat have high Attack stats, with Mawile's being at 85 and Volbeat's being at 73. Mawile already has more attack power. Mawile has high Defense stat at 85, but its Special Defense stat hangs at 55. Volbeat has a low Defense stat at 55, but a higher Special Defense stat at 75. Volbeat's the faster of the two, his Speed stat being 85 while Mawile's is 50.
For movesets and such I would recommend...
Mawile
Ability: Hyper Cutter
Nature: Adamant
Item: Leftovers/Shell Bell
Attacks:
-Iron Head
-Crunch
-Rock Slide/Ice Fang/Brick Break
-Rock Slide/Ice Fang/Brick Break
Mawile with the Hyper Cutter ability would work the best, just to make sure its high Attack stat won't be lowered by your enemy's moves. An Adamant nature would also give its Attack stat a nice boost. Your choice on which item, either would work fine.
Onto the moves...
Iron Head is powerful, is a physical move, and it'll receive STAB, 'nuff said.
Crunch is a real decent move and you'll find it handy on many occasions.
For the last two moves, Ice Fang and Rock Slide would be preferred. Ice Fang would help you against Ground-types who can KO Mawile, and Rock Slide will help you against Fire-types.
Ice Fang is an egg move however. Simply breed Mawile with a Floatzel that knows Ice Fang and your Mawile egg should know Ice Fang.
Rock Slide is a TM, TM80 to be exact, so it being on Mawile depends on if you have one on hand.
If you can't get both or even one of these moves on Mawile then Brick Break and another physical move like Return would make good replacements.
Volbeat
Ability: Swarm
Nature: Adamant
Item: Leftovers
Attacks:
-Substitute
-Focus Punch
-Moonlight
-U-turn
Other Moveset:
-Double Edge
-Confuse Ray
-Zen Headbutt/Brick Break
-U-turn
Swarm would be the best ability, since Illuminate won't do crap for it. XD And again, since Volbeat is a physical attacker, an Adamant nature would be a wise choice. Leftovers as an item would ideal since you'll want HP constantly getting replenished.
Onto attacks.
First Moveset:
Substitute plus Focus Punch would be the best thing for Volbeat. Use Substitute and then Focus Punch while your opponent is busy dealing with the doll. Volbeat has the speed to pull it off and if used with a bit of thought could work nicely. Both are only learnable by TM
On top of Leftovers, Moonlight is there to heal the damage done by Substitute.
U-turn happens to be the only physical Bug-type move Volbeat can learn, and I think it could be useful. U-turn switches out the user after it's used, but it deals real decent damage at the same time. So it could be used as a get-away in a sticky situation, or it could be used to deal damage, or both! U-turn is a TM as well.
2nd Moveset:
Double Edge is Volbeat's best attack, and your Leftovers will heal up the recoil damage.
Confuse Ray is an excellent move that can throw a great big wrench into your opponent's plans. Don't hesitate to use it.
Its your choice between Zen Headbutt and Brick Break, either would be fine, just take a look at what moves your team is lacking and go from there.
U-turn for the same reason as described above. =)