As someone with years of experience with bosses,I can offer some guides to help you.First of all,keep the Thunder.No armor can match its power.Secondly,keep the Jin.Jingle.Now,your M-27B1 is a bit outdated,I suggest getting the M-Z7B2.Fully upgraded,it beats the M-27B1 in both PvP and PVE.
When preparing for a boss,equip a Aid Kit,2 Hyper Clips,1 Big Health restore,the machine gun and a weapon with no speed penalty.Try to practice quickdrawing with the machine gun(check out youtube to see examples).
In Double Rages,dont camp,use the old run&gun method.In Serpent,camp in the corner opposite of the Rihno spawn.In Xmas Suprise,camp next to the ex-glitch spot.While fighting a boss,try to aproach it.It will try to do a close range attack,which you can easily avoid with your Thunder.
For PvP,get a better armor.If you have a sniper,like the R100 or the R700-AA,use it combined with the Atom(more HP then Thunder and also beam bonus) or the Perseus(a lot more HP but no bonus).Team up with other players,and make sure your player is fast while playing CTF/VIP.