Slow enemies with Q, then land on them W+E at the same time so they can't avoid your passive which is when you land 3 abilities then you apply additional True Damage.
Right after you land your W+E and successfully knock up the enemy then you should cast your R as soon as they are knocked up

(IMPORTANT NOTE: W does damage 2 times, if they get both damages they also get 2 passive stacks that's why you can also land W+E and still do a lot of damage because they will get 3/3 passive stacks)
(IMPORTANT NOTE: Remember that when someone is in your R then you will apply stacks over time so if they stand in your laser beam you will apply stacks on them over time.
Another important thing is that your R is a channel so it can be cancelled with any CC like Stun, Hook, Knock Up)

You should try to position yourself in such a way that you can still slow them and when needed use your W+E combo