Along with what was said above regarding Ele. Bomber you basically destroy PvE no problems whatsoever, not much funding needed as you can do good with just Magic Seals for Int or Spirit. beginning is pretty boring and lots of XXXXCFXXXXX, but you have amazing hitstun with electric element and electrocute procs ( use magic cannon and if it procs each hit afterwards will do its damage plus 2000 or higher damage). fire is strongest raw damage, but imo its not as good as dark or electric. Darkness has ultimate pierce, so you can shoot through any mobs. water is weakest raw damage, but has the best shield since it negates any 2 hits at max shield.
Regarding builds, all your skills are good and should be maxed if you like them. the only exceptions being chain lightning, crystal crash, and elemental rain(45). chain lightning and crystal crash are still acquired since they're pre-requisites to later skills.
Awakening is pretty solid, but its optional still.
At Level 55 you get Fire Road EX which basically burns the whole screen with a 5.5 second cooldown. it is amazing..
Don't really know about GMs since i have yet to make one, but from what i've seen they're more PvP oriented and look a lot more fun to play than EB's spam skill to win.