There are several issues with this player, so let's split this into manageable chunks:
Telling other players what to do is simply not acceptable. This rule should be stated clearly, and repeated before each session if needed. There are specific exceptions when telling others what to do is OK, like:
- Dungeon Master saying "guys, please, please don't split the party, it makes my life hard and I doubt I can manage"
- In-character noble or team leader giving orders to other characters, and roleplaying being frustrated when they wouldn't listen
If someone tries to tell others what to do, the only course of action that ever worked for me (and I DMed in a RPG club with little control over who plays my game) was to pause game and say something like:
Josh, it is OK to remind fellow players about some rule or character ability they might be forgetting, but please, do not try to tell others how to play. Nakravein is played by James, not by you. Please, respect that. I understand you just want to help*, but you are not helping by taking control and fun, from fellow players.
James, what Nakravein is doing?
* half of the time this part was a bluff on my side and I didn't really believe they want to help, but very useful bluff that lowered the chance to further antagonize problem player.
This sets borders quite firmly and gives control back to player who should have it. If "Josh" is still frustrated and trying to tell other players what to do, all you can really do is to repeat the process. You need to set borders and make it clear you will not allow them to be changed, and game will not continue with him acting this way.
Sometimes 1 on 1 conversation after session can be helpful:
Josh, other players wants to get their experience somewhere, and wants to have their fun and their spotlight. You are all equal at my table.
That should be a good start. Your club has some supervisor, I'm sure. If you ever need to repeat this 1 on 1 talk, invite said supervisor. You cannot tell problem player to behave or leave. Supervisor can.
A massive rules lawyer
If he is not respectful, then there is little you can do:
- Give ruling. If he argues, tell them that this is only a temporary ruling and invite him to talk about it after the game. Firmly set "no arguing at game table" as boundary you are not going to let anyone to cross. Reinforcing such boundaries - see previous section.
- If you can, arrive for your gaming session early. Tell that everyone who had issues with rules and rulings, or who expects to have such issues, is invited to arrive early and discuss them with you. Stress that once game starts, there will be no discussion.
- See How can I handle a player who pre-plans arguments about my rulings on RAW?
If he is respectful, employ him!
I have a player who is massive rule lawyer. When party is leveling up, I can let him answer most of the questions knowing that he won't make any rulings on his own, and anything he will tell other players will be strict RAW. Takes massive load off my shoulders. I can give him XP number for encounter and forget the math. I can ask him about spells I don't remember so I do not need to browse a sourcebook. This works great because I have help I need, he feels important (and rightly so!) without any chance to disorganize my game, and other players have someone who they can ask things if I'm busy with another player.
Designing encounters against his character
This looks little unfair. Let's use my last character as example: I made him excellent liar. If I would then realize that my Dungeon Master pumped up NPC to recognize lies more easily, I would feel frustrated. OK, of course you shouldn't let him to breeze thorough all encounters (like, giant spider didn't care about my speech, neither did traps), but if he built a character that is good against things he could reasonably expect to encounter in your campaign, and now meets things that are specifically designed to be especially effective against him, then he is right and you are nerfing his character. And it is hardly fair.
If you want to do that, do that in style. Let him win against unaware enemies. Make second or third encounter hard, as his character gets famous and there are in-game reasons for baddies to be prepared against him, especially. New bugbear tribe or new town shouldn't be aware and prepared any more than they are for regular adventurer.
If he optimized his character, it is only fair that some encounters will be easy for him. And when bad guys prepare especially for him, he should struggle, but he should receive it as a badge of honor: "I was so dangerous they made me their primary target! They were afraid of me!" - struggling for PC, but in a way that doesn't punish the player, or at least does not feel like it.
Take his Fly -> Polymorph -> Fall tactics. It is OK that enemies who never heard of it would fall for it. Given that this is not in the RAW, it is your job to decide if it is an attack (attack roll that can miss) or something that gives the target Dexterity Saving Throw. In no way this should be auto-hit, like a Cow Kill spell in Baldur's Gate. He should get full falling damage, hit or miss. And after few uses enemies will have hunting falcons (or worse) to discourage this, because enemies are not that stupid (unless, you know, he only attacked stupid ones that way - then they are).
And if you think it'll give him too much spotlight, then by all means, prepare situations where other characters will shine. Or struggle, for that matter. But if you will continue to nerf his character, his frustration will grow. Worse yet, his urge to force other players to optimize their characters will grow, because only when they are even there is a chance you will stop targeting and punishing him. Or, that's my guess based on Dungeon Master mistakes I made in the past.