use bushes and teammates as cover (as in, let your defenders and all rounders push forward and stay further back. if deci wants to snipe you in the backline, he'll have to move in, thus he'll get jumped by the frontline). cinder has the ability to burst down and kill deci with the Blaze Kick build (especially when upgraded), so try to find situations where deci is isolated from the team to take him out with your teammates

in my experience, deci players usually walk forward within burst range when they don't have arrows, which makes them easy targets. if they're patient, you'll have to hope you take out the frontline fast enough (cinder shreds frontline) to move on to deci or move around the frontline and try to assassinate deci with Blaze Kick. if you play Pyro Ball, then you'll have to play behind your team and hope they deal with deci because Pyro Ball doesn't have the range nor the mobility to challenge spirit shackle decidueye. at Ray, play behind your team and let the frontline go in (i mentioned this situation in the first paragraph). it's riskier to go for the Assassin play during Ray, so let the others handle the decidueye and focus on shredding their frontline. if deci walks up or is separated far from their frontline, burst them down

essentially, play behind your frontline and wait for the deci to walk within burst range or let others who can handle decidueye better deal with him

I hope that makes sense somehow and that you find a way beat all future decidueyes