Void is a very consistent DPS synergy because the attacks of allied units ignore a percentage of the enemy’s armor.
Assassins and Rangers are probably the best choices for physical burst damage. The former have increased critical damage, while the latter have a chance to double their attack speed for a few seconds.
Demons can burn the opponent’s mana and inflict true damage, which is particularly strong against tanks because it ignores the armor of the target and any other form of damage reduction.
Imperials double their damage, Ninjas have extra attack damage, and Wild units increase the attack speed of the team.
Finally, Blademasters have a chance to proc extra attacks, while Gunslingers can attack more targets.