Typically when you think of Engineer AoE you think of Grenade Kit. That is the same thought pattern this build uses so that you can hit large amount of enemies from a distance. This build maximizes on the Grenade Kit's ability to quickly attack so that you can tag plenty of enemies, while making the best use of Elixirs to stay alive. This build is incredibly simple and does what it is supposed to do very well if you stick to its theme.
This build invests fifty trait points into Grenade Kit. The ability to tag enemies quickly is mainly done with Short Fuse and Grenadier so that you are throwing as many grenades as possible to hit your opponents. Shrapnel and Scope are to accent the damage you deal and to encourage you to keep your distance from the enemies you are trying to hit. Lastly, Speedy Kits are use so that when you are trying to get to your next group of enemies you can pop your Grenade Kit off and on quickly to give yourself swiftness.
The other twenty trait points go into Elixirs and your survivability. Fast-Acting Elixirs is mainly there to make sure Elixir H and Elixir S are up as often as possible. Cleaning Formula 409 is there to give you some sorely needed condition wipe without having to use Elixir C.
Your utility bar should come as no real surprise. Grenade Kit should be equipped 99% of the time in combat to tag as many enemies as possible. Elixir C is a panic button when you get heavily loaded with conditions and know if you don't cleanse you will die. Elixir S is used to give yourself room from enemies when they start to focus you down, so don't be afraid to use it when you get a bit too much attention. Lastly, Supply Crate is pretty much your only option in this build as the other Elites are completely lackluster for this play style.
This build is incredibly simple in that all you need to concern yourself with is using every grenade skill as they come up. Grenade Barrage, your toolkit skill, is especially useful at hitting large clusters of enemies and tagging most of them. There is very little reason to use your main weapon set over the Grenade Kit, as none of the weapon options provide the same AoE and Damage as the kit.