Depends on what tier of materia you're aiming for. If you're aiming for III or IV materia, your fastest approach is to grab a group and just destroy everything in the area that is 48+. There are several areas like that. Otherwise, you could always find some friends to grind lv50 dungeons with a lot of trash mobs in a small area that aren't too bad to kill quickly (even in just the i55 gear), this being places like Brayflox. Depending if it's lv45-49 gear, AV or DD would probably work much more efficiently. However, if you're looking to just solo, then you really only have the option of going to beastmen strongholds, south shroud elementals, Garlean strongholds, or places like North Thanalan. Beastmen daily quests are decent depending on how far you are with them and what level of gear you're binding.

Now if it's low tier (like t1 materia), your options are a lot greater as you can easily find certain areas to AoE groups of mobs quickly.