Spitfire PvE: Max Blazing, Cartridge Expansion, Flash Bang, Freeze Grenade, Pierce Booster, Neil the Sniper. Bullet buffs cost a lot of MP, so you're doing everything in your power to increase your dmg and decrease MP costs. Using Muskets and maxing out cartridge expansion will conserve your MP. Use Crossmore and Freeze Grenade for crowd control. Flash Bang electrocution to increase dmg. Pierce Booster needs to be used in conjunction with Blazing Bullet to double the explosions for more dmg.
Mechanic PvE: for Males, max out Land Runner, Time Bomb, Tempester, Robotics, Secret Land Runner. Run around spamming Land Runners and Time Bomb, and use Robot Detonation. Before lv25, have fun suffering. For Females, max out Time Bomb, G-1 Corona, G-2 Rolling Thunder, G-3 Raptor, Robotics, and Gale Force. Mash the Corona skill for rapid fire, or let Raptor do the work to save MP. Occasionally throw a Time Bomb.