So Gearbox has added the Community Day 2013 skins/heads to the common drop pools for enemies, thing is they're bugged in many ways, one of which makes it easier to get them
Basically take any character you have, switch them to Normal mode and fasttravel to ClapTrap's Place, then run around smashing the ice chunks.
Once you take them all out, Save+Quit and reload.
You're likely to get at least 1 skin per run, but that isn't guaranteed, either way. Average time to get them all seems to be ~40 minutes of running around
Big thing to remember: Actually claim the skins before you save/quit. If you leave the pickup in your inventory, they will disappear
Tha skins/heads: