Defensive
Mawile @Leftovers/Rocky Helmet
Ability: Intimidate
EVs: 252 HP, 252 SpD, 4 Def
Nature: Careful
- Toxic
- Protect
- Super Fang
- Stealth Rock
Toxic+Protect is a staple on defensive mons. Super Fang to do damage and to speed up the opponent's death. Stealth Rock is always useful.
Offensive
Mawile @Life Orb
Ability: Sheer Force
EVs: 252 Atk, 248 HP, 8 SpD
Nature: Adamant/Careful
- Iron Head
- Play Rough
- Ice Punch
- Fire Fang
Both Iron Head and Play rough have secondary effects, so they get a Sheer Force boost. Life Orb because if you didn't know, a Pokemon with Sheer Force doesn't take LO recoil. Neat, huh? Ice Punch and Fire Fang are coverage.
Mega
Mawile-Mega @Mawilite
Ability: Intimidate--> Huge Power
EVs: 252 HP, 252 Atk, 4 SpD
Nature: Adamant
- Iron Head
- Play Rough
- Sucker Punch
- Swords Dance/Fire Fang
Iron Head and Play Rough are STAB. Sucker Punch is brutal priority. Swords Dance to boost up and then be able to gleefully spam Sucker Punch against anything faster, or its STABs to shatter slower Pokémon or switch-ins. You thought 678 attack was terrifying? After a Swords Dance boost it is capable of OHKOing nearly the entire metagame with neutral attacks, and 2HKOing almost every resist.