The best way to unlock the Minion Multitude Achievement is to visit the Dungeon with an ultimately imbued Arcanium Sword and Armor.

For the weapon, don't balance out your minion sacrifices so that you get a little bit of everything and instead focus on increasing the damage (brown minions) and just a little of the green minions to do massive damage with the power blow.

For the armor, don't balance out your minion sacrifices so that you get a little bit of everything and instead focus on increasing the defence (brown minions) and just a little of the green minions to regenerate life slowly (about 5 to 10%).

Don't take any minions with you and when you go into the dungeon and approach each group of enemies slowly so that you face only a few at a time. This way if your health gets low you can run away for a bit and regenerate.