Assuming we are fighting the Beholder at level 19, in a hypothetical world with very good rolls...
Strands of Fate for 1d8+10 damage, 11-18 damage.
Action point, Tendrils of Thuban for 4d10+24 (+6 con, +2 burning blizzard, +4 implement, 12 vuln) +2d6(Curse) for 30-76 damage.
Next turn, sustain minor for Tendrils, 1d10+24+2d6 for 27-46 damage, and just for fun lets say Wrath of Acamar crits with our standard action for 62 +4d6 for 66-86.
Every other hit by your allies did +12 damage during those two turns as well, thanks to strands of fate, but just looking at the warlock damage we're gonna see about 134-228 in that scenario in two rounds. Is the Warlock probably gonna miss one of those attacks? Well, probably. He does need a 12 to hit with Strands of Fate, and around a 10 for the rest. Still, with 4 other people in the group I don't think these things are going to drag on nearly as long as some people might think.
And hey, there's another thread about how I should have used a Ranger for that example to make it more convincing, as he would no doubt have done more damage without immobilizing and granting vuln 12 all to the Beholder.