Boegun supp go brrrt

idk just placing random skills and hope it works, I placed (N) as a sort of necessity imo and (O) optionals for other available skills that could be good for the user.

Blade- Warcry 9/10 for atk% increase. 9 for not overriding ths 5% extra buff. (N)
we could add Berserk too if tis suitable with recasting or if we need extra aspd / cr but 10 secs feels too short as a self buff to recast everytime as a supp so imo its better to skip (O)
Shot - Crossfire 10 can be useful to deal some dmg 
Twin storm 10 for dual pew pew and more bullets/shots (also shorter cd) since we wont be getting extra bullets from arrow bonus. (N)
Decoy for ampr + 1 min availbility. (N)
I wanted to put ailments but since BG gets a sadge chance reduction on it so i was hesitant to add it and skipped as a decision.(O)
could try Vanquisher 1 as an ender / 10 for reducing Twin storm cd by 5s and doing a bit of dmg(O)
Magic - MP charge and Maximizer 10 as a faster way to gain MP when we get ampr reduction from skills (such as Twin storm) (N)
Rapid Charge 5 for lowering MP charge cd by 1s.(N)
Finale and Impact placed for combo closing and skill cost halving if necessary for the user but isnt a must have. (O)
Martial - Chakra 10 for providing pt with ampr and a bit of dmg reduction
Energy control 1 can work well for surviving attacks/aoes but it would remove your Support auras in the process.
Halberd - Only skill id prefer from halberd is quick aura 10 cuz of the aspd. (O)
Godspeed wield is good if u need extra motion for yourself with the cost of your resistance,kinda risky if u want survivability, its also notable that you would have enough aspd and motion with agi so u might not need it either. (O)
Mononofu - Shukuchi for the dash (N)
Guard - Mirage evasion if we need to move whist casting (N imo)
Shield - We could take protection and aegis 10 from here to provide phys/mag res for pt members but if theres a tank in your party in becomes unnecessary (O)
Hunter - We could try taking Magic Arrow 10 for extra normal atk dmg and ampr (O)
Dark power - Eternal Nightmare 10 for def reduction on bosses (O but a nice addition) 
we could take soul hunter 1 as well if we require an i frame ability for insurance, we could also take this 10 for more dmging capabilities as well.(O)
Magic Warrior - Resonance 1 for faster MP filling while, espicially when we need to cast all the support auras together. (N)
Survival - I personally prefer to take Play dead 10  as a supp to revive and get into brrrr mode faster. its good as a stargem if u can afford it (O)
Support - Aid 10 to reduce revive timer of your beloved or not so beloved party members.(N)
Brave aura 10 for watk buff but reduces accuracy (N)
High cycle 10 for faster cspd but reduces ampr (N)
Quick motion 10 for faster aspd but reduces ampr (N)
we could take mana recharge if u want to provide a bit more mp regen for pt with the cost of self dmg reduction. (O)
Minstrel - Fantasy song 10 and Adlib 1 to give mp regen for pt members time to time. (N)

 I would say the placed skills on this build is a "necessity" to take and the rest mentioned are optional and if im not lazy ill label them beside what I wrote. (N) = "Necessity" , (O) = Optionals.