If you have master materias, then you'll beat them easy like me.
Otherwise, Ultima is the best. you should considered enemy skill, they'll help a lot. have 9999HP and 999MP with plus HP/MP materia is good as well. 4xcut/slash all are your choices if the enemy is weak.
KotR, BahamutZ, Hades, shadow flare can be used to enemies that is strong. I always use my physical cause it always done 9999 damage.
Cloud Ultima Weapon damage depend on Cloud HP. I only battle in monster arena on certain days, cause I get a lot of luck at that times.
(I always get no handicap, cure and status giver but I'll be find with Ribbon). Just keep out for no materia, level down, low MP/HP.