I'll be honest, I think I have 7 rounds played w/ the SE, but after seeing that, it makes me want to rethink that lack of usage. I'm assuming he can create his own FE's with Energy Drain?
Only with extremely fast cooldowns (and I'm not even sure about that), and more importantly, only if the target still has Shield/Barriers/or is a machine (Geth/Atlas/Turret). If your aim is to chain various Tech Explosions, there are better Engineers for that purpose.
Rather, Energy Drain to weaken Shields/Barriers, stun mooks for easy headshots (you don't need a Sniper Rifle to shoot them in the head, any weapon will do, I use the Lancer), and if someone happens to detonate a Tech Burst for you it's simply a nice little bonus. You also get Pseudo Tech Armor, which is really what makes the SE a Pseudo Soldier, but you have to be somewhat selective with your Drain targets, since you only get the Shield Bonus if you've drained Shields/Barriers from a target. It doesn't need to be the target you are currently engaging.
Incinerate is best used with Incendiary Ammo for obvious reasons, or you could try Cryo Ammo and get higher Direct Damage from it.
Decoy I take to Rank 5, since Phantoms and most Geth absolutely love it. The AoE shock attack is useful in choke points, even if the enemies don't engage the Decoy itself, they will still take AoE damage from it. Also useful for clearing Swarmers while you run away from them.
Weapon Damage passives all the way, including Headshot bonus at Rank 5. I do use liberal amounts of Hard Cover to get long-range Headshots with my Lancer, so incorporating that into your playstyle may or may not be relevant depending on which weapon you actually use.
I really don't recommend SMGs on him, because you could do better with a TSent or GI, in that case. Same with Sniper Rifles, really, might as well use those on the Salarian Infiltrator. Assault Rifles, Shotguns, and some Pistols though are all perfectly fine on him.
TL;DR take a heavy weapon and pretend he is a Soldier