either use moon shard + iron treads, or his E CD weapon + the glyph that says "the first enemy hit by E triggers the final explosion of R"
go fight something durable enough to survive you spamming spells at it
observe that each time your R spell explodes on the unit, it will jump backwards another ~125 units and hang in the air, just a little higher than it was before the last explosion
repeat this cycle until you can't even see what you're aiming at anymore
SUGGESTED COURSE OF ACTION:
either
change the R explosion so that it doesn't change their Z coordinate elevation; or
prevent the R explosion's knockback effect (but not its damage or freeze or other effects!) from being applied to a unit who is already frozen by R
STEPS TO REPRODUCE:
SUGGESTED COURSE OF ACTION: either