In Weerdough Plains, you can get an item whose name is something like "broken machine" (I forgot the name, but the icon is a silver orb). Keep this item in your inventory and go to the marsh world map and get struck by lightning. The item will turn into a tanning bed. Take the tanning bed to the boss angel for a new dialogue option. You will skip the fight with the boss angel and go to the "true final boss" which, once you defeat, will place you at 99% achievement rate. Then you can talk to Sukumizu in the warehouse for your last achievement and a nice surprise.

A special thank you to Suxa (the dev) for the tip in the "Bug Reports" topic.

Edit: Horheristo has a video up on Youtube which shows how to do this. Looks like he beat me to it, haha.