Decree of Pain can destroy all creatures, or give them -2/-2 until end of turn.
Extinction destroys all creatures of a type. Bye bye plants! Tsabo's Decree has the same effect, plus a bonus.
Forced March is a great way to get rid of tokens.
You can build up a lot of mana in mono-black, so In Garruk's Wake, Plague Wind and Overwhelming Forces should be usable.
Pestilence effects are a great way to clear a board - try Pestilence, Crypt Rats, Thrashing Wumpus, Pestilence Demon...Toxic Deluge will help as well.
Also, just a note - the tokens from the Guildmage don't grow from Avenger of Zendikar's landfall ability. The Guildmage makes Saprolings, and Avenger of Zendikar only boosts Plant creatures.