Fast Bowl

Even though the game categorises grenades and molotovs as throwables, these don't count!

It has to be a regular melee weapon - like an axe, hatchet, sickle, etc - that you aim and throw at an enemy to kill them. You can aim and throw melee weapons in the same way you'd use a bow hold right-mouse/LT to aim, and then left-click/RT to throw.

Either get this naturally through regular play (it doesn't consume scarce ammo to kill this way!), or otherwise the easiest place to get this quickly is with the druids in Casterfell Woods - where it helps that many of them carry melee weapons you can grab to use again.

It also helps to carry multiple melee weapons at a time so you can keep going if you miss, don't kill on the first hit - or if there are multiple enemies.