Go Berserk While Channeling

The main reason I chose Berserker is that he gets leech from his first lab. This means life and mana issues become a lot easier, and plugging in a Leech support gem gives you a total of 5% leech after killing something.

However, the most important reason is that he can spam warcries every two seconds and recover 25% of his life and mana instantly doing so. This just about cancels out many issues (like the traps in the Lab) and allows him to run around in tight quarters (again, like the Lab or certain boss encounters) to "charge up" his endurance charges before engaging the boss. Yet Rallying Cry offers a huge damage boost and Abyssal Cry takes care of pesky corpses and enhances clear speed considerably.

The next most important factor is his 40% more damage at the cost of 10% increased damage taken. It's worthwhile, mostly.

This is chosen because it allows you to benefit from two spells simultaneously.

Incinerate is used because it is a "direct fire" skill, causing damage almost instantly and actually hits, so you can leech and apply on-hit effects. It also has an inherent damage modifier that increases as you channel, so it's nice.

Arctic Breath is chosen first because it leaves chilled ground, and this not only lowers enemy DPS by 30%, it slows them so it is easier to kite around them and avoid damage.

Both Incinerate and Arctic Breath are projectiles, so damage can be augmented via one curse, Projectile Weakness, as opposed to having to use Flamm/Vuln with Incin/Bladefall and such. Flammability + Elemental Weakness wouldn't work too well with Firestorm because Firestorm does not have the damage output that Arctic Breath has without curse support.

Firestorm offers the best DPS on paper, especially when you take increased duration and such into account. However, it requires time to build up and it has a set outcome. Arctic Breath does less damage with just one projectile, but it can shotgun and deal considerably more damage depending on the circumstance. However, there are two main issues that need to be addressed:

The first is sudden bursts of enemies. Devourers surrounding you, strongboxes, plummeting ursas, and leaping enemies can surround you and kill you with burst damage. Firestorm does not "shotgun" against them, but Arctic Breath does. Firestorm also doesn't have it's full DPS potential within the first channel effect, Arctic Breath does. AB offers utility, survivability, and damage while Firestorm offers only damage. Twenty plus enemies attacking you simultaneously can be countered with AB, and more enemies closer to you just means shotgunning is gonna happen, so enemies die more quickly.

Enemies that are dead generally deal no damage. Usually.

This is used in tandem with Mokou's Embrace to offer a considerable DPS boost, utility for moving (Leap Slam), additional leech, considerable damage, and proccing two CWDT setups to guarantee certain effects.

Level 1 CWDT gems are used for everything.

Enfeeble (level 5), Temporal Chains (level 5), and Flesh Offering are used in one setup to lower enemy DPS and combat potential as much as possible. Offering type doesn't matter, I just prefer red, but it clears up some corpses along the way. Offerings are swapped out for Frost Wall in some circumstances.

Blood Rage (level 7) is used in another setup to guarantee some attack speed (Leap Slam) and a source of generating frenzy charges. Frenzy charges are mainly used for the 4% more damage per charge (12% more total with the inherent charges) for some DPS boosting. However, the more attack speed is also helpful for mobility purposes. Blood Rage is kept somewhat low because we want a lower duration (in case the degen hurts).

Lower level CWDT gems are used so that EOI and enemies proc everything often, thereby applying linked skills to many enemies or keeping the duration on Blood Rage topped off most of the time.

I'm planning on adding in another support or spell, but haven't worked out which is best yet.