I lost Dooku in an arena battle to this but it worked in my favor as well as I had attacked their Sid earlier and he had the DOT and just a sliver. I was able to ignore him and hit Phasma first and knock her out, their Sid activated and died. Two down in .5 seconds.
This is how I expected it to work when I started. You have to be careful with people low on health with DOTs still active but I've always been a fan of killing someone without them knowing they are already dead.