Assassins are currently in a bad spot in the Dota Underlords meta because of the prevalence of Knights and Warriors, which counters them directly. You’d usually want to avoid them right now.
ASSASSINS + ELUSIVE
Units: Treant Protector, Nature’s Prophet, Templar Assassin, Phantom Assassin, Queen of Pain, Bloodseeker, Sand King, Slark
Synergies: (6) Assassins, (3) Elusive, (1) Demon
The benefit of the lineup is that you reach the (6) Assassins damage bonus while making your TA and PA tankier thanks to the (3) Elusive bonus.
It’s usually a good idea to replace Nature’s Prophet with Lone Druid in the late game in order to get the (2) Beast bonus thanks to Sand King (this also wouldn’t lose you the (3) Elusive bonus). Moreover, Lone is a much better tank than Natures thanks to the Bear summon.
ASSASSINS + PRIMORDIAL
Units: Tiny, Morphling, Queen of Pain, Templar Assassin, Phantom Assassin, Bloodseeker, Slark
Synergies: (6) Assassins, (2) Primordial, (1) Demon
Tiny with the Primordial synergy is one of the best Frontline units in the game, and with this lineup, you can easily get it because Morph is a Primordial Assassin.
The lineup matures at 7 units. In the late game, you have a choice – against lineups that deal a lot of melee damage (Warriors, Knights), your best bet is to go for (4) Primordials to make your Assassins tankier. That said, even with the (4) Primordials buff you’ll usually struggle to defeat top-tier Knights and Warriors strategies simply because your Assassins don’t deal enough damage to said tanky Alliances.