Mage Slayer Feat.
Note the line, "Taking this feat reduces your caster level for all spells and spell-like abilities by 4."
Therefore, this is not a good idea. You're effectively a 9th level Sorcerer in your build in the OP (which, by the way, is already written as a 12th level character, so I'm a little confused, but I'm assuming you just put it up there because you're really close to hitting level 12). Reducing your caster level by 4 will make you effectively a 5th level Sorcerer, you'd only cast up to 2nd level spells, no more
Polymorph for you for a long time. The Mage Slayer feat is for non-casters.
Are you asking for ideas on what sorts of things you might be able to do to be more effective and survive and thrive? Because I'm sure some of the fine people here could offer you useful ideas.
Would you be willing to give up
Polymorph for another spell? I'm figuring you want spells that increase your combat effectiveness while still allowing you to cast spells. Some suggestions include:
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Bite of the Warewolf (SpC p.29)
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Greater Invisibility (PHB p.245)
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Sandform (Sandstorm p.119)
Another alternative would be to start casting
Polymorph on your allies instead of yourself. Turns out when you start playing more of a party support-seeming role, in general a specific player tends to get less negative attention from a DM.
Slight nitpick: While Mage-Slayer is indeed a terrible, terrible idea for the OP, the caster level reduction wouldn't disallow his higher-level casting. It would just put his effective caster level for all spells at 5, reducing damage, duration, and most importantly, forms available for Polymorph (can't 'morph into something with more HD than your CL).
Haggling with the DM doesn't seem to be an option, if I follow the OP's argument. Like, at all. Also, while Polymorph is a broken, broken spell, as we all know quite well, Bite of the Were-X is not so far behind. The sheer numbers given in the spell description will probably put off the OP's DM.
Greater Invisibility is a valid option next level, but doesn't do much to increase damage output. I'd consider it, if it were me, but for pure combat effectiveness, Greater Invisibility is only of interest to sneak attackers IMO.
Sandform is really more of a utility spell than a combat buff, isn't it? Where it buffs, it's more of a defensive thing anyway. So again, no real advantage for damage output.
Moreover, choosing something else over Polymorph doesn't change the fact that caster enemies will just try to dispel him all the time anyway.
To me it seems the OP's character is intended as a melee machine. Going around to buff others makes him effectively the sucker for the simple reason that he has taken levels in Monk and Enlightened Fist instead of Incantatrix and War Weaver. Leave buffing to more dedicated buffers. Putting Polymorph on somebody else will also likely be less effective a use of the spell than 'morphing himself. Finally, dispelling somebody else isn't really any harder than dispelling the OP's character. I'd say his modus operandi so far is sound from a theoretical standpoint.
I cannot see an easy in-game solution, and out-of-game talks haven't worked so far and seem to hold little promise, should you try again, OP. Therefore, the last bit of advice I can give you is the following:
Think from a different angle: You've already "won". Most fights revolve around your character to a large degree: You're able to stomp the opposition unless they spend considerable resources on getting rid of your buffs. Non-caster enemies have no good answer for your polymorphing scheme, and caster enemies waste their actions countering your buffs (which you can re-cast) instead of laying waste to your party with save-or-die and battlefield control.
In most fights, you can either lay the hurt down - or you're at least doing an excellent job of tanking the opposition's spell output. It's a bit like a counterspell in advance, with the option of trashing the enemy if he doesn't try to dispel first chance he gets! Should you ever acquire a Ring of Counterspells (or take the Reactive Counterspell feat yourself), the first Dispelling attempt will even be blocked. And fire resistance really shouldn't be hard to come by in some form.
Far as I'm concerned, you're good, you're sitting high and dry, you're being effective. Sure, you could be even more effective with more specialized tools (items, polymorph forms). But by no means is your character being picked upon in a way that renders him impotent!