You need to finish the game first, or at least reach the final area and have a horde size of 50 (all command upgrades and the command helmet). The goal is to get them all to 400% which requires the late game golden equipment.
Take 50 browns (it's easier to see equipment on them) and, if you took the destruction path, head to Empire City and start busting open crates (there are loads around). If you went with Domination it's a little easier as Blacksmiths also make the weapons after you finish an area, you're looking for gold but they appear silver/crystal in Nordberg (due to the snow). Simply move from blacksmith to blacksmith sweeping your minions in with .
As with the last game it's handy to take your minions out of the portal one by one until you see the % drop below 400, put that one back and then guard marker the others (hit ), thus all the minions you take out will be sub 400 and those already maxed out won't get in the way of them picking things up. It isn't an issue early on as most minions are picking things up, but (for example) when you only have 3 minions left to max out the other 47 can block them rather badly.