Here are a few choices that I think would pair well with Mega Absol.
Cofagrigus @ Leftovers
Ability: Mummy
EVs: 252 SDef / 252 HP / 4 SAtk
Calm Nature
- Will-O-Wisp
- Rest
- Shadow Ball
- Energy Ball
First up is Cofagrigus who can pull off a mixed wall set with WoW and SDef EVs. It's great bulk allows it to tank a large number of hits, and it resists Absol's weaknesses. Shadow Ball is STAB while Energy is to get the drop on lots of tanky Pokemon like Rhyperior and Swampert.
Roserade @ Black Sludge
Ability: Natural Cure
EVs: 252 HP / 240 SDef / 16 Spd
Calm Nature
- Toxic Spikes
- Synthesis
- Giga Drain
- Sludge Bomb
Another option is Roserade who can tank a huge number of special attacks while also setting up some Toxic Spikes to harm your foe. Synthesis is recovery while Giga Drain and Sludge Bomb are STABs and provide good coverage.
Chandelure @ Choice Specs
Ability: Flash Fire
EVs: 252 SAtk / 252 Spd / 4 Def
Timid Nature
- Shadow Ball
- Trick
- Hidden Power [Ice]
- Fire Blast
Last off is Specs Chandelure who can provide some offensive support for Absol with a good variety of powerful Special Moves. It would also allow you to switch over to Psycho Cut on Absol which is a far better option. Trick screws over a lot of opposing walls who could potentially tank Absol's attacks.