SorO was referring to Plane shift, which can indeed banish creatures to a plane of the caster's choice and is a 7th level spell...
Yeah, missed the paragraph at the end of Plane Shift that says you can use it on unwilling targets. However it does say that it banishes them to a random plane--not one of your choosing. Depending on the target and the plane they end up on, they might be able to pop right back on their turn.
@Tenacious:
IIRC, there were a couple links to averaged out monster stats by CR way upthread. I took a look but the formatting was shit-terrible. Might give parsing it another try.
Fireball may not be a single target spell, (neither is sleep for that matter) but it's got the highest damage of any 3rd level spell--which is why I used it for my initial comparison. Like I said, Sleep affects a higher HP total than most damaging spells when cast with the same level slot--regardless of whether they're single-target or area spells. As for other control spells, most of those allow a save or require an attack roll. Sleep is one of the few that requires neither. If you're the only target in the radius and you're not immune, your HP is the only thing that can save you.
Since Sleep doesn't actually deal damage, there is the problem that you can't "stack" it's effects across multiple castings. This makes Sleep better as a follow-up spell rather than an opener, but the fact remains that on any given round, Sleep is more likely to take a single target out of the fight than most damaging spells. Depending on your save DCs, there's also a pretty good chance that it would be more effective than casting a save-or-die. No risk of your enemy rolling good on a save if they aren't allowed one in the first place.
After a quick check of the MM, it appears that enemies
are supposed to have way more HP than even the toughest party members encountering them. While even a 1st level Barbarian caps out at 17 HP, (1d12 +5, assuming he somehow manages to get 20 Con at chargen) a lowly CR 1/8th bandit has 11 HP, (2d8+2) and the CR 2 bandit captain has 65 HP, (10d8+20!) which is just farging ridiculous.
While this does make one-shotting enemies with Sleep pretty unlikely, it also makes attacks that do HP damage even worse. I suppose that does make save-or-dies the better option, assuming you can rely on your target failing their save. Seems like it's better to target saves than AC at any rate, since AC appears easier to buff up to high levels.
Hmm, also makes positioning a bigger concern. Casting Sleep into melee would be a good way to get a party wipe, since even at full health your teammates are likely to have much less HP than the thing they're fighting.