You won’t be able to extract Dark Elixir until your Town Hall is upgraded to Level 8, but these late game units usually have some special skill or trait to make them worthwhile.
Hog Rider--These boar-mounted assault troops have high health, high damage output, and can leap over walls. Very good attack troops.
Minion--Not very powerful, but Minions are fast and cheap. Their flying ability protects them from cannons and mortars.
Witch--You can think of witches as a form of summoner. Rather than attacking directly, Witches spawn skeletons to attack for them. So long as the Witch is alive, she’ll replace any skeletons destroyed in the battle.
Golem--Golems are very similar to Giants: large, high HP, and slow movement speed. Golems have one other trick Giants lack--Golems split into a pair of Golemites when killed, and the new units continue attacking.