Mega aggrons generally prefer Impish or Relaxed (trick room, also makes aggron us slower than Aegislash). Since such aggrons like to spread status like t-wave and toxic, a bulky spread is suggested:
248 HP/ 252 Sp.Def/8 Def
Offesive variants prefer Adamant or Brave natures. Recommended set would be something like:
252 HP/252 Atk/4 Def (with 0 IVs in speed if trick room, but good IVs if Rock polish).
Hope I helped!