I dunno what’s RNG about the casts though, it’s right after they cast voidbolt.
Cast voidbolt, kick Abyss stun. DPS add.
Edit: You say it yourself here Horrific Visions: Your progress? - #57 by Adrot-tichondrius, that Resto druid is OP for doing horrific visions. Since you’re able to do 4-mask without being stunned, then it’s a matter of gear that’s the problem.