Dragon Lord

Dragon Lord is a flying boss that comes on 27 waves, from wave 21 to wave 119 (Acts I–IV).

Waves: 21, 24–25, 27, 30, 35, 37, 39–40, 44–45, 47, 51, 54, 56, 59, 64, 71, 74, 77, 82, 89, 99, 103, 107, 113, 119
Element: Fire
Movement: Flies, 1.4 tiles/s
Armour: None
If it gets through: 3 lives

How to beat it

It is a fire creep: water towers deal it 1.5× (Chill, Frost Tower, Blizzard, Frost Bolt, Glue, Permafrost, Frostbog and Ice Shot and 13 more). Nature damage is weak against it (×0.75 on a boss).
It flies: it ignores your maze and heads straight for the goal. Towers that target flyers first: Cluster Mortar, Ice Shot, Focused Beam, Lightning Tower, Storm Caller, Overcharge, Tempest, Poison Tower, Plague and Spore Cloud and 6 more.
A boss that reaches the goal costs 3 lives, so give it the longest walk past your strongest single-target towers.

Dragon Lord on Normal, solo (the first wave of each act it comes in)
Wave	Act	Per lane	HP	Regen / s	Kill gold
21	Act I	1	3,739	–	50
35	Act II	2	5,055	–	15
64	Act III	3	7,598	–	35
99	Act IV	3	13,559	–	49

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.