Imo, I think it should be possible to destroy the target completely (meaning you deal enough DPS for the raid) even with the minimum iLvL, although at such level one must be doing a heck lot of DPS burden, so it's... understandable at best. I'll just share to you questions I'd ask myself for times like these. Hopefully this helps!
  • What's your rotation? How about a video of it?
  • Have you tried a different opener? (Something like 1-sen opener or 2-sen Hagakure opener instead of forcing it to 3-sen)
  • Do you use Hagakure often? (It should be the priority over using Midare whenever it's off the cooldown)
  • How many Hissatsu: Shiten can you pull off?
  • Do you keep Kaiten + Higanbana up?
  • Do you always manage to use Kaiten in every Midare Setsugekka?
  • Do you always keep every 2 buff and 1 debuff (Yukikaze) up?

Edit: iLvL 310 eh? It should be more than enough. Keep trying!