Nightmare/Doppelganger

Strong early game DPS, rounds 1-3 especially, but also quite squishy when used as a lonely melee unit. Too expensive to risk having it die halfway through the round.

Early game can be held with just two doppelganger but this is a desperate, low-income starter build which will require additional value on round 3, and is also not suited for offlane. Can be rough if enemy is able to send 2 wyverns, or if one ends up tanking way more than the other. Ideally you'd request to use your dual-teammate's full-sell before round 6.

Doppel however truly shines when paired with unit like peasant, as it enables a bit higher push than otherwise possible and is often a better gold-sink compared to more mass. Full sell as early as 4 (it tends to clear very fast, unfortunately leading to unhealthy amounts of aggro). Try to have it join the fight a bit later than other units and probably do not push past 9/3.

It is OK on 5 but ill-advised rounds 6+. It dies fast to champion and likewise to big send. Hopefully at this point you can exchange it for backline DPS.

With Seraphim it's a strong secondary candidate for revive. Mostly during mid-game though.