One of the mods that I use is called MMM (Martigen's Mutant Mod). If you thought Super Mutant Behemoths were large and a PITA, well the mod adds Gargantuans which are even larger and meaner.

If you are very very unlucky, you can even discover them out in the middle of nowhere. At first, I didn't notice the big orange thing as it was obscured by the monorail track support. It wasn't until I started my attack on the Super Mutant Brutes that I realized what it was.

For my 3rd attempt - I simply ran far far away without engaging that group.