It's not your 1st mech bad, it's your 2nd, trust me, i was supposed to be rank 14 not rank 15 but my 2nd mech is too weak(rare and epic trash) hold me in rank 15, now i have the same problem, still stuck in r12-11 because my 2nd mech dmg is too low and energy cap is horrible so you should upgrade your 2nd mech first, so if your 1st mech ded, your 2nd will take care of them instead, and use "scope" in arena, make your mech become scope and very high chance you won't down to r17 again :)