Actually Cerberus and Doomtrain have statdef x4. But if you're looking for malboro tentacles and remedy+, you probably only have cerberus

You can mug malboro tentacles then run away, if you can't survive bad breath. Usually you'll get about 4-8. plenty for doomy.

You have to have med level+ to get remedy+. that's the easy one, really

They show up the most often on the island closest to heaven. It's really annoying for me to find, so just go north east a LITTLE ways off of where that huge forest is, and keeping looking for one that says 'island closest to heaven' in the status screen.