Slime
Creatures: Green slime, jelly ochre, grey ooze, and jelly mustard
Strategy: Usually not posing any threat, slime's only notable attribute is that the harder slimes will require +1 weapons. But, you'll probably have them beforehand anyway, so it really doesn't matter. They can also poison you, but you'll most likely kill them before it ever poses a threat.
Spider
Creatures: Baby, huge, giant, wraith, phase, and sword
Strategy: These creatures can be found all over the world, but especially underground. Individually, spiders won't pose a huge threat, but when 20 small spiders attack you, you've got a problem. It is often worth it to cast the level 3 mage spell fireball at your own party. This will do 6d6 of damage to all creatures. Yes, your own party members will take some damage, but it will kill all the small spiders, and it is often preferable to have a slightly damaged mage over a near-dead and poisoned one. When fighting normal spiders, just use warriors and prepare to cure the poison. When fighting phase spiders, it is best to use archers.
Troll
Creatures: Ice, snow, giant, spectral, spirit, and so on
Strategy: Trolls are really not difficult if you know what to do. Though they can pack a punch, hit them from afar until they fall over unconscious. Immediately cast a Melf's minute meteor spell (which creates 20 throwable meteors) and attack them once. The meteors will hit, and they will fall over dead. Next time you encounter a troll, make sure your mage does not attack. Then, when the troll is down, attack once more with the mage. Repeat as needed. If you are fortunate enough to have ample fire arrows, those work as well.
Umber Hulk
Creature: Umber hulk
Strategy: Easily one of the most difficult early game enemies, umber hulks will charm and confuse enemies up the wazoo. The easiest way to dispatch them is to open a door, then step back. If you've done it quickly enough, the hulks will not see you. Once the door is open, cast web and ensnaring Roots or any two other hindering spells, then cast as many area damage spells as you have. If any break through, and only one or two will at a time, hit them with magic missiles and send in your fighters. In case they do get spells off, keep your mage (or whomever you use for dispel) a good way off. If you are hit with a spell, grab your mage and simply dispel the effect. Repeat until all the umber hulks have been destroyed.
Vampire
Creatures: Male, female, fledgling, mature, patriarch, and so on
Strategy: Casting many aggravating spells, vampires can pose a problem. Most importantly, vampires will cast level drain, which is especially annoying when cast on mages and priests, as it will force you to rememorize all their spells once your cure them. However, to prevent this, either wait until you're affected and cast the priest spell lesser restoration or cast negative planar protection beforehand. If your cleric would lose the lesser restoration spell from being hit, it is important that you cast negative planar protect on the priest, so you can heal the rest or your party. Otherwise, once you kill any vampire, looks for its coffin. You will need a stake to put it to final death, but it will be worth it, as you will receive an extra 9,000 experience points from the already generous amount given when it died originally.
Wolves
Creatures: (Greater) werewolf, (greater) wolfwere; and dire, dread, worg, and winter wolf
Strategy: Customarily dealing small damage, the only wolves of consequence are the werewolves. As long as you have +1 weapons or better (are we sensing a pattern here?), though, you will rip through them like fresh meat.
Wyvern
Creatures: Baby wyvern, wyvern
Strategy: If you somehow don't kill them quickly, they have a poison attack that can get very annoying. However, they are extremely easy to kill and should be easy for anyone to dispatch once you find them.
Yuan-Ti
Creatures: Yuan-ti, commander, priest, mage
Strategy: These guys look very similar to salamanders, but are much tougher. The salamanders, who are very physically weak, posed no problems and were easy. The yuan-ti are also very physically weak, but have very strong and incapacitating magic. They are rarely alone, and if you don't concentrate on them, they will bring the effectiveness of your fighter to that of a mage in melee combat. They have low magic resistance, so hit them with spells, arrows, and, of course, good ol' hand-to-hand slicing and dicing.
Zombies
Creatures: Zombie, ju-ju zombie, sea zombie
Strategy: Looking much more formidable than they really are, zombies don't do much of anything but lope around. They can disease you, though. But to prevent this, simply attack from a distance. Once they get close, move father away and resume attacking.