Claimed by darkness; slayers are the bane of anyone standing in their way.
Dark Slayers are virtually impenetrable to physical attacks: aside of the Musketeer's Sniper Shot, nothing does more than scratch this hulking monstrosity. Nonupgraded archer towers will make them laugh at your pathetic attempt to kill them. The normal strikes of a Paladin do zip, and the 500mm Big Bertha, ignoring half the armor level, barely damages it. Even a fully upgraded Rain of Fire can't kill one. Barracks troops quickly fall to the Dark Slayer's powerful attack. Use the highest level magic towers possible, and Death Rays... or the Dark Slayer shall surely live up to his name.
High-level magic towers and/or instant kills are essential to punch through the Dark Slayer's armor. Use mages level 3 or 4, combine with Paladins to kill them. Do not use Barbarians to block the Slayers, as they will be killed very quickly and unable to inflict much damage through the Slayers' 95% armor rating.
However limited Reinforcements or low-level barracks are against the Dark Slayer, they are still able to stall it for a few seconds. The more time standing in one place means more time exposed to magic towers. Since Reinforcements have a fast respawn rate, you can use them to keep a Dark Slayer in range of a magic tower for longer.
Where feasible, one hit kills such as the Arcane Wizard's Death Ray is preferable. If you lack money for the upgrade, an Arcane Wizard is still very effective against Dark Slayers. One hit from the Arcane ray will drop its health by a visible amount.
Use the Sorcerer Mage's curse to reduce their physical armor, making them more vulnerable to artillery and archers. Elementals can hold Dark Slayers for a fair period of time.
Reinforcements are only good for blocking one, as even with all upgrades, the melee attack deals exactly 0 damage before the Dark Slayer's powerful attack kills them both. The ranged attacks only deal 1-2 damage per throw.
if you upgraded Spiked Armor on the upgrades, it can reflect damage back to Dark Slayers even though Soldiers can't deal damage to them.
Paladins with holy strike can harm them very quickly. A fully upgraded Paladins is even better, because of armor, healing light, and holy strike. This helps Dark Slayers to fight them for a long time. Just make sure you had other towers for Shadow Archers.