New Engineer
Flamethrower is a pretty easy way to level for most people. Has range, aoe, burst, blind and knockback all in one. The first ten trait points if you’re going for fast leveling should be in Tools to pick up Speedy Kits, which nets you easy perma-swiftness. After that it really is preference. You could stick with flamethrower or if you like spamming keys grenades once you have the master grenadier trait(playstyle is not for everyone) or go for bombs which are also pretty nice since smoke bomb in pve is like godmode for the duration on slow mobs.
To level quickly you want two things: Power to kill foes faster, and Swiftness to travel across the map faster.
Get a minor Sigil of Bloodlust; it costs less than 1 silver and provides a significant damage boost at lower levels.
You should get Bomb Kit and Med Kit ASAP; Bomb Kit will be your main source of damage and Med Kit your main source of Swiftness.
Other important skills are Elixir Gun, Flamethrower, Rocket Boots, Healing Turret and Grenade Kit: Elixir Gun is your general tool box, Flamethrower is good to farm events with lots of mobs, Rocket Boots will get you across the map a little faster, and Healing Turret will become your default healing skills once you’re level 20; at lower levels Grenade Kit is mostly useful for the debuffs of #3 and #4.
Put your first 9 points into Explosives for more damage. Once you’re level 20 respec and put 10 points in Tools for Speedy Kits for perma Swiftness. Put your next 10 points in Explosives for more damage and bigger bomb radius. Put your next 10 points in Firearms for more damage.
Once you’re level 40 put 10 points in Tools and 20 in Explosives for more damage, bigger bomb radius and 10% more damage. Put the remaining points either in Firearms or Tools for more damage or Alchemy for Vigor.
Once you’re level 60 put 10 points in Tools and 30 in Explosives for Shrapnel/bigger bomb radius, 10% more damage or 20% less recharge and Grenadier. From now on Grenade Kit becomes your default range weapon set and Bomb Kit your default melee weapon set. Put the remaining points either in Firearms or Tools for more damage or Alchemy for Vigor.
Kill with Bombs (But this is 2nd Tier, so you might not be able to grab it off the bat)
Move with speedy kits + Rocket Boots
Sustain with healing turret + super elixir