Gasoline

Tactics
Gasoline does as described; the major note is that enemies killed by the Gasoline burn damage will themselves burst into flames and add more Gasoline effects, which stack when they overlap. If you can hit an enemy hard enough to kill them, you can set off a chain reaction that will help you thin out crowds.

Item Synergies
Gasoline stacks really well with other effects that occur on a kill—like the Will-o-Wisp and the Frost Amulet. Likewise, it helps makes DoT attacks—like the Acrid's poison more effects, since you can trigger them and then back off.