Zombies with hollowed out chest cavities that are packed full of (possibly also undead) plague rats!
I like the Iron Golem/Red Dragon thing. The version I did, was the dragon's lair was in the old stereotypical volcano with an escape tunnel carved out that connected with the still active and lava-full cone. Two iron golems that were shaped like sea serpents swam around in the escape tunnel, guarding it. 20d6 hp / round healing for each of them as long as they did their job and stood guard. Mean stuff.
Otyugh should bury themselves in refuse, and just pop up the eyestalk and tentacles to fight. Let's you grapple and drag under prey while protecting your body at the same time.
Trolls: swamps, yo. On fire? Take a dip - fire's out. Splashed with acid? Wash it off.
Ankheg: burrow underground, ambush prey from below, grapple 'em w/ improved grab and drag the prey off into your tunnels to eat at your leisure.
Giants: For the love of god stop throwing rocks. Learn to use big javelins or bows or something. The rock thing is just embarassing. Unless you're a stone giant, because then it's okay.. you know.. being stone giants and all.
Gnolls: ditch the scale mail for a chain shirt, pick up Scent, Run and Endurance. Always travel in groups of at least 5. Don't attack when it's light out or well lit. You have Darkvision - use it. Draw prey into dark areas (or herd them there), and put out existing enemy light sources. Flank constantly - never let prey fight you one on one. Always make it so he has to choose which gnoll to attack and which ones he wants tearing him appart from behind. You're pack creatures - use pack tactics.
Goblin: Get shortbows, train with Worgs, and take one level of your favored class. Worg-riding Goblin Rogue/1s with shortbows are frickin mean, particularly in large numbers.
Clay Golem: get in an anti-magic area. You are then invincible.
Rakshasa: find some way to get around that annoying blessed crossbow bolt nonsense. A CR 9 creature should not have an instant off switch like that. Particularly not one that any given 1st level cleric with the right domains (elf, time) can get through.
Any Incorporeal Undead That Has A Draining Attack: use guerilla tactics - attack out of a solid object (floor, wall, etc), attack for a round or two, do some draining, then retreat. Repeat until foe is dead and/or one of you now.
Tarrasque: eat stuff. then sleep for a while.
Thoqqua: Get in your opponent's square. Try and grapple. Provoke as many AoOs as humanly..er.. thoqqualy possible. Touch them. Make them touch you. They more chances you have to pump out that 2d6 fire damage the better. If things are looking grim, tunnen straight down. Then poke up at the other guy's feet from below.
