žestok

1. (of a fight, argument, criticism) fierce; violent; harsh
   Posle utakmice je izbila žestoka tuča. — A violent fight broke out after the match.
   Ministar je dobio žestoke kritike. — The minister received harsh criticism.
2. (of drinks) strong; hard
   Ne pijem žestoka pića, samo vino. — I don't drink hard liquor, only wine.
3. (of pain, heat, cold) intense; severe
   Probudio me je žestok bol u leđima. — I was woken up by a severe pain in my back.
   Ovog leta vlada žestoka vrućina. — There's intense heat this summer.
4. colloq. awesome; wild
   Sinoć je bila žestoka žurka! — Last night's party was wild!

It usually means fierce or intense, though it can also mean harsh or cruel; the usual direct equivalent of 'cruel' is okrutan.