I don't really think this falls under the exploit section, however this bug, per say, causes your Deadly Poison to tick for 1,400 damage.
During the Kael'Thelas encounter in phase two, pick up the Infinity Blade. The weapon gives a passive 5% magic damage increase. The 5% increase effects each stack of Deadly Poison you have up, mixed with talents to improve your poison, causes it to do nasty damage.
1) Get Infinity Blade
2) Stack Deadly Poison
3) ????
4) Profit.