I finally figured out where Druid shines and it breaks the game. The key is the Ravager class.

Ravager class provides:
[20% chance to BLEED enemy upon CRIT]
[33% chance to FREEZE + BLEED on MARK] <-- Key

For Druid:
Riposte/Ravage ability will always MARK on enemy.
Gain +10 MP per MARK on enemy (max +30 MP per turn at 3 MARKs)

Since the ability Riposte/Ravage cost -30 MP = Free.

As you can see, you would be spamming Riposte/Ravage to bleed and freeze enemies. Enemies could not do anything but be frozen and bleed to near-death (bleed is % of CURRENT health so bleed will never kill enemies). In the end of combat, once enemies are near death, you just do 1 ability to knock them out, lol.

Note: To get Ravage ability, go to Library's Inspect quest - Passcode is 9853 then activate the quest.

So I decided to try out a boss in Colosseum keystone room with Temple's Monster Power Level at maximum. lol
322k health...good thing I have something surprise for this Victory Lordgod pal...

lol...
I used Immolation mantra (damage over time) to kill him

Level 20 is when you get to choose a class. At level 20 and on, you would be doing this. Turns out, you just had to beat the boss in Cellar Door to unlock class advancement. I just happened to level to 20 last time. Now, I would advise against raising Monster Power Level because you might come across a monster with Resistant affix. They are immune so you have to fight them the normal way. Also, Skeletons are immune to bleed. For them, keep them frozen first. Start immolation and refresh it when 1 turn left.

Enjoy!

After playing with multiple different Druids with Ravager class. The best setup is to focus all on VIT so you would be using Reckoning mantra (HP --> Damage). Every turn, you lose 10% of your health to deal damage with this mantra. Just spam Q and keep an eye on your health. As soon as it turn green, start mantra, refresh if your health is still green. This becomes more "spammable" if you come across Lizard Garb (artifact, equip it in your Wardrobe) that provides +5% MAX HP per turn but lose 30% ATK (which is irrelevant to mantra, lol...) This halves the 10% HP loss from Reckoning. Kililng becomes quickier with repeated use.

In addition, be a Huskian race for that 5% chance per turn to gain 25% MAX HP. You want your health bar to go up as many as possible to repeat Reckoning.

Spamming Q with Reckoning in between, I ended up killing enemies much faster than Immolation mantra. Go with Axe weapon mastery to gain little speed in killing the enemies (10% chance to do 1% current HP damage)

At start, things might feel slow to you. Bleed does take a while to reduce enemies' HP below 50k health. But at high level with super VIT stat, Reckoning mantra will do more and more damage over level, killing enemies below 100k health faster and faster.

An example of what I did using this build. in 2 hours and 36 minutes, focusing all points into VIT and doing only Ranked Matchmaking as soon as it is available at level 30 (I also do portals and Arena when I can). MAX Monster LvL via Temple.

ELO over 6k and climbing. Unstoppable against non-Resistant opponents
See 2 Hours and 36 Minutes.