Rimeshroud

Rimeshroud is a boss that comes on 25 waves, from wave 14 to wave 114 (Acts I–IV).

Waves: 14, 20, 24–25, 28, 30, 33, 35–36, 39–40, 43–45, 49, 54, 57, 67, 76, 80, 87, 94, 97, 105, 114
Element: Water
Movement: Walks, 1 tile/s
Armour: None
If it gets through: 3 lives

How to beat it

It is a water creep: nature towers deal it 1.5× (Tesla Coil, Thunder Spire, Ball Lightning, Poison Tower, Plague, Spore Cloud and Sorcerer). Fire damage is weak against it (×0.75 on a boss).
It regenerates, so damage has to come faster than it heals. Slowed creeps regenerate at 50% of their rate.
A boss that reaches the goal costs 3 lives, so give it the longest walk past your strongest single-target towers.

Rimeshroud on Normal, solo (the first wave of each act it comes in)
Wave	Act	Per lane	HP	Regen / s	Kill gold
14	Act I	1	3,230	2.8	39
33	Act II	3	7,352	2.2	20
67	Act III	3	14,467	1	44
94	Act IV	4	17,881	1.5	39

HP grows with the wave and the difficulty; in co-op every lane gets its own. Numbers from the game's data when this page was built.

The Last Maul

Co-op tower defence for up to 8 players. Free to play, nothing to install.