I may be the only person holding onto random crap they'll probably never use that ALSO makes the effort to skill up that same random crap because "you never know", but just a reminder that skill-up dungeons skill up ANYTHING that has the skill of a card, not just the specific card they display – so on top of all the Beach REM cards, you can also skill up:

• All forms of Light Metatron including chibi & her equip, as well as Mito from Heroines
  
• Fire and Dark Chesters
  
• non-Beach Kurone
  
• non-Beach Fuu
  
• regular, non-machine UVO Goemon, if anyone still uses him for farming (saves you having to evolve the descend version, which has a different skill)
  

There's also the skillup monsters for those cards above, I guess, if you want to be really specific – Light Chaser, Black Bird Bomb, etc – but I doubt many people are THAT invested.

I should probably just sell all this crap, but oh well!

Somewhat less crap you can also skill up:

• Beach Lakshmi has the same skill as Awoken/Revo Lakshmi, so you can skill them up too – one of I think only a handful where low-tier cards share an Awoken card's skill.
  
• non-Beach Fujin, which otherwise without py's is kind of annoying to skill
  
• non-Beach Pandora, I guess if you don't run Awoken/Revo for some reason
  
• L/L UVO (the newer one) of non-Beach Tachibana
  
• edit: non-Beach Urd, props u/esperstron/ – forgot her