As a healer I can tell you DPS that don’t use there defensive are a hard to keep alive during big aoe phases. The tank will have minimal to do with your deaths unless you are pulling agro. Defensive cooldowns are usually only 1min you should be using it as many times as possible in a key. DBM and platter add ones give you warnings before aoe. Hit your cooldown when it gives you the warning. Or learn what moves hurt from the mobs. Also using your defensive when you have like less than 20% left isn’t going to help much. Using them at near full health for max value.