Overlord

A horde of 50 minions with a battle strength of 400% is necessary for this achievement. The fastest way to accomplish this is to warp to the Ruborian Desert town grab 50 browns and ransack the town, the minions will automatically choose the best gear. This will have to be repeated about 5-6 times before all the minions are fully equipped.

To get the Ultimate Horde achievement, all 50 minions need to be at 400%. This means you need all 6 minion caps and an Arcanium helmet maxxed out so you can summon the extra 15 minions, and you need to be in the desert so you can get the best equipment.

When you summon minions from a hole, the most powerful come out first. Here's what I did to get the achievement:

I went to the desert, walked to the minion spawn on the bridge just next to the town, locked onto the summon hole with the left trigger, and summoned my browns one at a time. When the first brown came out, the Horde's power was at 400%. When I summoned my 32nd minion, the horde power dropped to 399%, and if I had kept summoning at that point, the number would have kept dropping.

I then put all my minions back in the summon hole.

I then locked onto it again and summoned 31 minions. These were my strongest and were all at 400%. I walked about 20 feet away, pressed Y to set a guard marker, and then walked back to the summon hole.

I locked on again and pulled out the next 19 minions. These were the weaker ones that needed equipment. I walked 20 feet in the opposite direction, set a second guard marker, and put the 19 minions there.

I then walked back over to the first guard marker, locked onto it, and pressed B 31 times to call my 400% minions back to me. Finally, I went to the summon hole, locked onto it, and held down the right trigger to send all my 400% minons back to the hole. This kept my 400% minions safe and out of the picture.

I then held down B and called the 19 weaker minions to me. I entered the desert town with them and pillaged, walking all the way from the first section (which has the side area with 2 exploding beetles, which is where most of the equipment is), past the area with the magma beetles, and up to the rest of the town. My minions smashed everything I saw. I got to where you see Assel, sent my 19 minions back to their hole, and teleported to the tower. This ensured all the desert barrels reset.

Auto save, back to the desert.

This time, when I locked onto the minion summon mound, 44 of them had 400%. I broke them into 2 groups again, put the 44 strong ones back, took the 6 weaker ones with me, and repeated the process again.

Took a total of 10 minutes. Once my little group of 6 was at 400%, I went to a summon hole, called out the other 44, and as the 50th minion came out, the achievement was unlocked.

Another way to do this is, at the end of the game, in the room where you have to re-obtain the minion hives under the tower, there are a lot of boxes containing the gold helmets and spears. Run through this area with 50 brown minions breaking every box and picking up the items. Rinse and repeat and the achievement should be yours in about 15 minutes.

*EDIT*
To get 50 minions at once you need to find all minion tower upgrades (bringing your max to 35) and also forge a fully imbued arcanium helmet, therefore bringing your max minions to 50. thanks to Epsilon Theta for pointing it out.

I personally had problems with using browns in the Ruborian town as Magus suggests; I always wound up getting stuck at 398%. Greens worked much better for me.