this is for a br/ra tree which is assuming you are not planning on playing RA mainclass at all.
I have Weak Bullet added in the very, very rare occasion you might need to Weak Bullet. Keep a Holy Ray handy, don't need to grind it, just carry it incase you might have to be the one to weak bullet.
Some skills on the tree that are added to just max out all your points after CO's and Class EXCube tradeins are RA Shooting up skills and BR Jreversal cover and maxed out Step Advance, level 3 is quite enough, but you litterally have nothing else to really put it into. you can put some stuff into katana tree incase you might need it (Anga fighting, etc.)
Your main source of damage mobbing is Penetrate arrow/Million Storm.
For bossing, you'll always want to throw 3 chase arrows out before your combo, you can shoot them before teleporters too. Chase arrows do not hit anything on cast, they're stored.
Main combos are with stocked up chase arrows are like: [JA]Banish Arrow> [JA] Final Nemesis [JA]> Uncharged penetrating arrow [JA]Banish Arrow> [JA] Kamikaze Arrow (Full Charge)> [JA] Kamikaze Arrow (Uncharged) 2x [JA] Banish Arrow> [JA] (Crafted) Tritt Shooter
A reminder, you don't have Massive Hunter, so you can get hit out of every PA you do, including Final Nem, so position yourself out of harms way.
mag you want to build around 150-160 r-atk and 40-50 dex if you're starting out, or you can go raw r-atk. its really up to you. If you plan on using Red weapons for awhile you might need that dex to average out the damage. You basically feed r-atk weapons and if you want to lower the dex feed it furniture.
Units it's really up to you and what you're planning on getting. Brisas/Saikis are still very, very good and are in the top7 units still for general purpose. Fahren units you get for Zieg are good starting units that you essentially get for free.
bows you want to get 2 at the end game.
Edel Arrow: http://pso2.swiki.jp/index.php?%E3%82%A8%E3%83%BC%E3%83%87%E3%83%AB%E3%83%BB%E3%82%A2%E3%83%BC%E3%83%AD
Photon Booster Latent 3 it gives a 34% damage bonus during Rapid Shoot. (If you want to be lazy you can keep it at Latent 1, you only lose 4% damage)
and
Divine Raikou. http://pso2.swiki.jp/index.php?%E7%A5%9E%E6%92%83%E3%83%A9%E3%82%A4%E3%82%B3%E3%82%A6 Latent 3 makes the Cooldown of Rapid shoot down by 18% (You're essentially without rapidshoot for 10 seconds)
You can also keep an Orbit bow handy to latent 3 for PP regen when you're standing around for 10 seconds.
Also, I recommend getting a bow camo, so you're able to switch bows and maintain Chase Arrows.