this matchup HEAVILY favours zoo in my experience. Unless you get perfect draws and they play terribly zoo is going to stomp handlock, abusive seargeant and dark iron dwarf make easy work of giants and they still have 4+ minions left to keep bursting you down.
Hellfire all well and good but it doesn't get rid of divine shielded minions, golems deathrattle and hurts you which helps the zoo. + they will just play 2+ more minions next turn and you're back to where you started.
shadowflame -> great spell, requires a body. Any half decent zoo will make sure you don't have one when they see you are handlock,costs 4 mana and requires a combo so best case scenario you can't play it till turn 6 (sunfury/watcher + shadowflame). at this point if you've done nothing you're down to about 10-12 hp and they can hapiply pick up where they left off. If you've been using earthen rings etc you're probably at about 17-20 ish, but oh look they have 2-4 new minions next turn and you have no answer to it.
Reading all these comments the only thing i can think of is that in the last month zoo has seriously evolved from what you are talking about.
don't get me wrong i'd love to be proved wrong and have some one teach me how to beat zoo, but as things stand it's such a bad matchup for me i can hardly be bothered to play it.