80%+140%+180%+220%+260%
So you did 5 times the onslaught ability. This ability gives you mastery, not the siphon stacks. The siphon stacks decide how much you'll get.
For example:
You had about 100 Mastery at the beginning. You'll gain +80% after the first onslaught ability which will lead up to 180 Mastery. You used the second ability once you had 140 stacks of siphon which result in 180 Mastery + 140% of 180 = 432 Mastery. So on and so on...
I recommend you to use the onslaught ability more often, even at later stages because you'll boost up way more.
Only use the basic if you have low health to heal up.
If you see that it is a close fight, try to use a health/speed set with health primary.
Use always the event ability on GL Kylo.
Also Rey get the Memory buff which will reduce her damage but this can be ignored if you stack your damage up. You should easily survive her hits and heal back up at later stages.