So, for generic improving your damage in raid advice, there's actually a few things.
the first is be luckier
This is sort of a joke, sort of not; at the end of the day, the highest parses generally require good luck, especially with our current tier set. Getting an extra proc of SC reduction and a ton of SS resets can have a drastic effect on your damage output, for example. It's just the nature of how dealing damage in raid can be. You can consistently reach good parses even with bad luck, but it's always important to remember how much of an effect luck can have since raid encounters are quite short compared to a whole dungeon run, so some luck can influence a lot. With that said...
If you're aiming to optimize damage, you'll want to start tracking juggernaut and sudden death stacks while playing the execute build. While getting sudden death itself is luck based, you can give yourself the highest chance of keeping juggernaut going by spacing out your executes pre-execute range without wasting stacks of sudden death. If you've played Arms, it's a similar idea as to what they aim to do, just simpler on our end. Going into execute phase with juggernaut fully stacked can do wonders for your damage output.