Complete 100 Rank 3 Hollow Missions in multiplayer. This is a grind, no way around it, and it's tedious. Grind to Level 200 first before doing this, then pick a HNM (the Level 151 Kobold HNM in Selvendys is the easiest, in my opinion, and it'll get you a ton of snazzy trenchcoats to dress up your waifus) and then kill him in a single OSS combo. Repeat until done. This should be one of the last achievements you do. Don't do this until you can comfortably kill a HNM in a single OSS.
At level 250 with a full-offence build, I can nearly oneshot him with a single cast of Nebularaid Empress.
EDIT: You can potentially combine this with the Aegis Shield implement, which requires you to finish 30 Hollow Missions at Risk 1 or less. Use the Divine Sword Dragon Slayer, also from an implement, which sets your risk to 0 as long as it's equipped. You also need 15 HNM kills for Epetamu, so you work those into this as well. Downside is that it'll take you longer to kill the HNM, upside is you get two things done at once.
You'll also get ores from mission completion, which you can use for the 200 upgrades achievement.