Quote from FoxfireInferno >>

Also, remember teamwork.  If one monster can't hit him, have two gang up, one using the Help action so the other gets advantage.

I'm going to assume those 20s are in DEX and INT.  That accounts for an AC of up to 20.  Where are the other 3 points coming from?

He can only Bladesing twice between rests, so having more than two battles between rests is also an option (and considered the D&D SOP, really).

But yeah, I strongly suspect that pair of 20s is the culprit more than the class itself.  Personally, I would have everybody redo their characters with Standard Array or Standard Point Buy.

 Discussion's probably over, but the 3 points come from Mage Armor. Bladesing let's you add Intelligence to AC so 13+Dex+Int.

Tbh I’ve never got why the subclass called Bladesinger was attached to the wizard instead of the bard (then again College of Swords exists). If I were to retune it, though, I would:

1. Ban multiclassing with it. This just prevents headaches with optimiser shenanigans.
2. Either halve the movement speed bonus or remove it. You’re not a monk.
3. Change the AC bonus to half, round up. Signing up to be a frontliner should have at least some inherent risk of getting hit.
4. Change the Song of Defense feature to 2x (maybe 3x) the spell level instead of 5x. Your frontline longevity doesn’t need to be that big if you have ways to leave the area immediately (misty step).

Open to feedback / being torn a new one, but it’s my personal opinion.

You can do what you want to in your game. I don't believe there is a need to nerf the subclass the way you have. Better to just say you can't play it in your game. Would make your life easier.

Quote from Wysperra >>

You can do what you want to in your game. I don't believe there is a need to nerf the subclass the way you have. Better to just say you can't play it in your game. Would make your life easier.

 Agreed.

Quote from Kailaidescope >>

Tbh I’ve never got why the subclass called Bladesinger was attached to the wizard instead of the bard (then again College of Swords exists). If I were to retune it, though, I would:

1. Ban multiclassing with it. This just prevents headaches with optimiser shenanigans.
2. Either halve the movement speed bonus or remove it. You’re not a monk.
3. Change the AC bonus to half, round up. Signing up to be a frontliner should have at least some inherent risk of getting hit.
4. Change the Song of Defense feature to 2x (maybe 3x) the spell level instead of 5x. Your frontline longevity doesn’t need to be that big if you have ways to leave the area immediately (misty step).

Open to feedback / being torn a new one, but it’s my personal opinion.

 
Do you also ban other, immensely superior, optimiser shenanigans like 2 paladin dip with Warlocks who can stack Divine Smite with Eldritch Smite to completely nuke anything? Or 2 dip to Fighter for Action Surge (2 Fighter, 2 Warlock, 3+Sorc, can cast Eldritch Blast 3 times in a turn, by like 12th level this is 9 beams for a nova 9d10+45 force damage). Seriously, of all the "optimiser multiclassing" to ban, why ban the weakest one? O.o 

So do you also ban other movement speed bonuses that are not Monks because you do know other classes can get these too? Doesn't make sense. It's just 10 ft and the reason they have it is because bladesingers are still not best for front-line and need the movement, similar to how monks and rogues do. Skirmishing is still necessary and that requires more movement.

AC is helpful but not everything. The idea is it will help but half, which is only like 2 points, is very insufficient. Even with the feature as it is, the bladesinger is still not best to tank at front line. Bladesingers can get great AC, but still requires a lot of investment and even with these the squishy HP makes them not great at tanking. AC won't help them resist a paralysing spell, or a a fireball to their face. The primary purpose of the AC boost is so they can wade into melee more freely without spilling all their slots on Shield spell just to survive a few rounds. 

And Song of Defence is fine. The damage mitigation is not great even at 5x, that's 45 damage with a 9th level slot, and at that level you're facing enemies that can dish out 60 or more in a single strike or near a hundred with a cone blast or such. So for the same slot that could be used to warp reality, you're reducing a single attack and still getting damaged anyway. Wizards are the squishiest of classes, so for one that is going to be more at risk in combats than other wizards, they really need something to use as an emergency - and no, the immense magnitude of variables means Misty Step is a woefully inadequate solution. Song of Defence even at 5x is still less "haha, that barely hurt" and more like "I was able to avoid getting instant-KO'd". 

Bladesingers can be OK at front line but sparingly - their ability to fight there is very resource draining as it is. They are best as skirmishers or duelling single enemies. If all you get is one or two fights a day, sure they're going to be rather spiffy. But you're meant to have multiple encounters a day and so a wizard - whose magic is going to get used up both in and out of combat - is going to get severely drained of resources very quickly. Think less of what a Bladesinger can do in a single battle and more about what they must do over a day. 

As Wysperra said, you do you. But it seems you'd be better at just banning the subclass entirely than nerfing it to pointlessness. 

Glad to be torn a new one; that’s actually very helpful to know on how bladesinger actually works.

What one word command could get them to do that?

Quote from nickignome >>

What one word command could get them to do that?

 “Cease” or “Stop”, maybe?