Well first, I just want say nice moveset. Personally, if I was using Polish, I would drop Substitute and go for a third attack. Also, maybe drop Power Up Punch. Its kind of too weak to rely on even with boosts.
Anyways, Aggron is a defensive MONSTER. It could pair well with anything that can''t take physical hits itself. Fairy always pairs well with Steel as it absorbs Poison and can take resists itself. Fairy can tank fighting but doesn't particularly care for Fire types.
Aggron will want to stay alive and be able to perform so keeping it healthy with either a Wish Passer or a Cleric to clear status is recommended.
Togekiss @ Leftovers
Trait: Serene Grace
EVs: 248 HP / 252 SpD / 8 SpA
- Toxic / Thunder Wave / Heal Bell
- Air Slash / Moonblast
This set is meant to tank hits for Aggron and keep him healthy. Wish and Protect can help keep Togekiss healthy as well as stall late game. It also scouts out Choiced Pokemon. Toxic for Wish Stalling, TWave for ParaFlinch, Heal Bell to keep burns and paralysis away. Air Slash is a great STAB move, but Moonblast is great defensively.