The Melee class has the greatest amount of defense. The Ranger class can deal damage at distances, but needs to stock Ammo. The Mage class can deal damage on normal distances. The Summoner class can deal damage when not attacking, through the summoned minions, but has the least amount of defense.
Other game items can deal damage: Explosives and one-use Weapons. Stat buffs include: damage, critical strike chance and attack speed. You can buff these stats by using consumables and trinkets.
Note: the Player is considered "In Combat" when they are either dealing, or are being dealt damage from a Hostile monster.
Damage done from the environment, or to breakables is not considered "In Combat" for the purposes of some effects, such as the speed boost provided by the Traveler Cloak.