I know that most people don't use it. they just stand around and attack the boss until they die. But dodging is VERY important. I killed Dezeadon and Arcane Warlock at full power solo, and would have lost if I didn't dodge.
The basics on how to make the dodge acually happen. press W twice to dodge forward, press S twice to dodge backwards, press A twice or Q once to dodge left, and press D twice or E once to dodge right.
First, use it the way you are supposed to. Are they shooting stuff at you? Dodge! Trying to stab you? Dodge!
Second, you can use dodging for movement. If they are about to lauch AOE attack, and you are playing as a slow character, you can use dodging to get out of there fast. If you are at low health, you can also use it to escape. If you are slower then the opponent, you can use it to avoid getting stabbed over and over again. Or even if you just want to get back in the battle faster.
Third, (and this is a bit strange) but if you dash into an opponent, you can acually knock the boss back, or even knock them over. This is an alternative to dodging, and more effective if done correctly, because the shockwave will not hit you, some ranged attacks will not hit anybody while they are knocked over, and they are easier to hit while they are down, because they are not moving. If you are going to try knocking them over, use the forward dodge.
P.S. If the boss says something in the battle and/or a forcefield appears around the boss, they are about to use a high damage AOE attack. When that happens, GET OUT OF THERE FAST.