Power Catalyst
The Spear Catalyst: Elemental Empowerment stacks on you as the fight runs, and 
Empowered Empowerment makes the top of that stack worth reaching.
Fire and Air for the multipliers, the Catalyst line to build and cash the stack. Damage that ramps, on a spec that wants to hold ground.
Why these traits
Elemental Empowerment gives you stacks just for being in combat; 
Vicious Empowerment adds more when you disable or immobilise a foe, 
Evasive Empowerment adds more when you evade, and Empowered Empowerment makes every stack do more — and more again at the cap. 
Elemental Epitome is the second engine: comboing grants you an aura, and granting yourself an aura grants another stack, which is why the sphere's combo fields matter to a damage build. Fire and Air underneath are the usual package — 
Power Overwhelming for power, 
Bolt to the Heart for finishing, 
Burning Precision keeping targets burning for 
Pyromancer's Training and 
Persisting Flames.
Gear reasoning
Dragon's and Marauder: 
Scholar is flat power and ferocity, so the prefixes are the only place vitality comes from. 
Force on both sets, and 
Impact for the extra 7% against stunned and knocked-down targets — the same disables Vicious Empowerment pays you stacks for. 
Bloodlust and 
Fireworks both reward a long fight: power off kills, and strike damage after a weapon skill with a 20-second-plus recharge.
When not to run it
Everything about this build climbs: the Empowerment stack, Bloodlust, Fireworks. In a fight that is a string of short skirmishes you never reach the top of any of them.
If your squad hits and resets, Power Evoker and Power Tempest (Spear) both start faster.