Instructions
Activate a trinket. Only a handful of trinkets in the game break the Polymorph spell, such as the Engineer-made Minor Recombobulator, and you must have the trinket equipped in the "Trinket Slot" before using it. Look for trinkets that indicate they dispel "Movement impairing" effects in the "Use" section.
Damage the target to remove the Polymorph effect. You can only damage enemy targets, so this method will not work if you wish to dispel a friendly target of Polymorph. Any damage done to the target will instantly remove Polymorph.
Cast a dispel ability on a friendly target. All dispel abilities in the game remove the Polymorph effect from friendly targets, although generally only casting classes have a dispel ability for friendly targets. For example, a priest's "Dispel Magic" can dispel Polymorph.