Resonance of Fate 4K/HD Edition (HP)

The best way to do this is in Chapter 16. Once the chapter starts, check the guild board and one of the missions will be a Rebel Suppression mission. This mission will take place on Level 4 and the last exclamation point will be a battle against the Elderly Man and his minions. I would recommend saving your game before the fight and every time you nearly run out of Bezel Shards just in case a stray enemy grenade gets into his range. Just shoot him with the Machine Gun and you'll level up the MG really fast. After hitting him with Full Scratch Damage, escape from the battle and re-enter it. It should take about an hour and a half to get to Level 100 with the MG for all characters.