If you choose fire as your main force mastery:
- max fire force serie (fire imbue)
- max flame-body serie (increases physical damage - very good for a str build)
- max flame-devil force serie (passive skill, increase physical damage too...)
Of course, you can get fire protection & fire shield series, but those are not necessary and costs many SP (it looks like you don't have much).
Don't delevel lightning mastery, just keep it for grasswalk-flow. You can also delevel lightning imbue, but you won't have many SP back IMO.
YES you can choose Ghost-Walk phantom lvl 1 (useful to escape while doing jobs, and to grind faster).
... and have a look to the stickies in the 'Characters and skills' section, you'll find some good guides
