Spitfires are ****ing terrible for soloing if you want MP efficiency. Grenade reloads will cost A LOT of MP, bullet enchants won't let you clear an entire room in one enchant (at least two and a half depending on what enchant you're using), and their overall PvE skills are almost STRICTLY party damage support (see Neil, which makes you vulnerable to monster attacks, Photon/Napalm that only locks down enemies for your team, Freeze Grenades to lock enemies, Blazing Bullet to group enemies, Crossmore to group enemies).
Mechanics are definitely your choice if you want STRICT PvE. There is no argument here if you bring Mechanics to the table as your choice for strict PvE for Gunners. MP efficiency is absolutely terrible at the beginning with only Corona, but once you have access to transform it into Thunder or Raptor, as long as you keep transforming, MP is less harder on you. They're able to lay down robots that auto-target for you and the likes.
Also, piqued, not peaked.