I recently decided to try out the N7 Demolisher.
Build: Max Everything bar fitness.
Wep: Striker Assault Rifle (level 3) - Stability Mod / Thermal Clip as I'm unsure if the grenade-ammo type actually uses piercing.
Equip: Warfighter III (Assault Damage+Grenade)
Ammo: Cryo (free rifle snap-freezes) or Disruptor (Electro Arc madness).
Now, before anyone comments - I don't have the gear that is deemed 'standard' for regular Engineer play, so I picked whatever I felt I'd be comfortable with... And boy is it fun being a walking detonation machine with infinite ammo and a rifle that knocks back everything in sight, with anims & sounds that make you feel like a mech with an automatic rocket launcher.
Explosive ammo rifle obviously has its own perks (knock mooks out of cover with trick-shots and can also fire up and down over steps or high rise terrain to hit people in the head even if you can't see them. Lack of any CD other than Dispenser means I can use whatever I want. Dispensers are placed right on top of other ammo crates for mass grenade pickup between roaming.
Cryo is lazy as you snap-freeze targets after 1-3 shots but it's fun watching icy explosions.
Played it in Gold and Plat (mostly with me arriving a few rounds late) and still comfortably gathered solid scores despite being behind and focusing more on trying to support people than actually go around killing things with my not so epic level III equipment as a first time Demolisher.
Going to stick with this for a while and see how it goes.