I think Lord Zadoz nailed it- this player is a strong tactical player who metagames out the wazoo, and you're more of a narrativist/storyteller. Those two playstyles NEVER mesh well.
For what its worth, I agree with you Slaygrim- this player is an immature, whiny jackass who believes his playstyle is the only way, and you should conform to pre-established hard rules so he knows EXACTLY what to expect in game. Don't get me wrong- the game needs rulese, but knowing that every encouter your face will consume roughly 20-25% of your resources makes for a DULL game. I also suspect this guy has some serious inferiority issues and frustration with real life, and wants to cut losse and be "DA MAN" in game. Nothing wrong with using the game for that, but there is if he's being a jerk about it and disrupting the group and gameplay. In short, talk to this guy, or get rid of him.
Also Slaygrim, how does this player know the levels and power of these encounters? Are you telling him at the time in game, or after the game? I'd be REALLY irked if a player stopped a fight complaining that it was too powerful, and how could I give them an encounter so high above their CR. This is one of the things I REALLY hate about D&D 3.x- the assumption on many player's parts that things have to be exactly RAW, and if they aren't, the DM is somehow a bad DM. I've dealt with whiny players like this before- and every time it stems from entitlement issues, the player wanting to be the "badass" and ALWAYS look super-cool. In short, immaturity.
For the other posters- look at what Slaygrim has said. He said he told the party BEFORE this big fight that there was an ally wizard pretending to work with the BBEG, who would help them- but the party forgot this. In addition, the 19th level wizard was otherwise occupied with a ritual, and wouldn't be part of the first fight. This is not a case of a DMPC- this is a case of a DM using an interesting plot twist to keep the narrative and story entertaining. I mean, God forbid he deviate from the "kick open the door, kill everything, loot it" playstyle.

And his encounter with the bandits who were basically fodder so the assassin could scope out the party's abilities is GOOD DMing- it makes logical sense in the world, for the assassin, and makes the story more interesting. It also doesn't sound like the other players have any problem with this DM- just the jackass player.
Finally, the iron golems. He said this tomb has been sealed for thousands of years, and the only logical critters in there would be golems and undead. I'm guessing this dude is a blaster wizard, with almost no spells that are utility or buffing. This guy built his wizard to be useless in some situations. Now, how many times has this guy blasted happily away, killing dozens of enemies to the fighter's handful? Yes, the spotlight was off him for a while- and thats fine. In the course of Slaygrim's campaign, it made logical sense to have a tomb with undead and golems- and in that case the wizard assumes a different role- that of support. Its not like EVERY encounter for the rest of the campaign will be golems and undead. I'm not argueing that players shouldn't have fun, but this guy needs to seriously grow up and consider other people's feelings and the fun of the group, rather than just himself. Complaining like this and whining is also highly disrespectful to the DM and other players- he's basically saying "you people don't know what you're doing, and I'm going to make myself the center of attention again by pitching a hissy fit."
I really feel sorry for you having to play while this guy DMs Slaygrim. I can guarantee he took this personally based on his immaturity, and he will take it out on your character. Don't retaliate by being an ass- think of the good of your group and be the bigger man. If he gets nasty or overbearing, just excuse yourself from playing and tell him when he grows up to let you know.