Of course, this is classic Warcraft. Back then, Medivh's backstory hadn't been fleshed out yet. He didn't have an assistant named Khadgar and he didn't work with Garona. Medivh was just a crazy and evil wizard who opened a portal into the stygian abyss and summoned demonic orcs to destroy Azeroth. He needed to be destroyed and it was your job to do so. There was none of that 'guardian' stuff, no possession by Sargeras, no reincarnation as 'the prophet,' no Draenor, etc. Things were simpler then.
Let's start off by talking about the big bads. You'll fight skeletons, slimes, and spiders along the way. But nothing compares to these three enemies:
Fire Elemental: You'll fight four of these creatures in the mission, but thankfully not all together. They're extremely fast and attack by exploding. Their attack damages one of the units surrounding it, but it's hard to tell which one. Three explosions kills a knight. Two explosions kills everybody else. The last one you fight is paired with a daemon.
Daemon: Before the Burning Legion, there were these 'daemons.' Nobody knows where they came from or where they went after Warcraft II, but Medivh has one in his pentagram room. The daemon is the strongest unit in the game in terms of HP and attack. He can kill your archers, footmen, and clerics in one hit. You'll need to use Medivh's corridors and your archers' long range to your advantage in order to minimize the damage.
Medivh: The big bad wizard himself will continuously summon spiders until he's out of MP. The spiders will eat through the door and attack you from time to time. Not only that, but he has an incredibly damaging fireball attack with a really quick firing rate and long range. It will definitely take all of your remaining troops to take him down.
The map looks like this and your formation will be key. The numbers below correspond to each skirmish. It's important to enter each skirmish with the right formation and then completely heal all the units with your clerics afterwards. There is no time limit on this map, so you can take as long as you want.
1: Fire Elemental
|Be careful of the explosions.|
Send one knight to lure the fire elemental back to the rest of your knights. Have your cleric heal the knight that's being damaged by the fire elemental and you should defeat it with all of your knights intact. Continue to the very bottom of the map. You can leave your footmen behind if you want. They don't do anything in this mission until the very end.
2: Fire Elemental, 3 Skeletons
|Muster your forces.|
Set up a chokepoint at the bottom of the map. Archers in the back, knights in the front. Two clerics to prepare to heal the knights. Take one knight along the path and lure the skeletons and fire elemental back to your chokepoint.
|The fire elemental approaches.|
Once again, you should be able to clear them out with no casualties.
3: 9 Slimes
|Set up your formation once more.|
Proceed past the small room and set up another chokepoint at the tunnel. This time, you'll be facing some slimes. There are three slimes clinging to the bottom of the map and six more on the right wall. You can lure the slimes back to your choke point in two different waves.
Once again, your knights might take a little damage, but nobody should be killed. Take your clerics and use far seeing on the upper right corner of the map. You'll uncover a den of spiders.
4: 10 Spiders
These spiders don't run out of mana. Luckily, they're just as weak as the summoned spiders Medivh's been throwing at you. The way to them is unblocked, so just take four knights and clear out the den. Heal with the clerics and bring the rest of your troops to the spider den.
5: Fire Elemental, 3 Skeletons
|Here comes another fire elemental.|
Set up a formation just south of the spider den and lure the skeletons. The fire elemental might come too. If everything goes right, you won't lose any troops. Now all that's left is Medivh, a final fire elemental, and a daemon.
6. Fire Elemental, Daemon
|Use the maze to your advantage.|
At this point, the stupidest thing you could do is rush in and attack the fire elemental, daemon, and Medivh all at once. You haven't come this far just to throw it all away.
|The knight will serve as a decoy.|
Bring the rest of your units down to the walled area just outside Medivh's chambers. Don't break any of the doors. The goal here is to send a knight into the room with the pentagram and lure the fire elemental out. Then, while it weaves through the corridor, you shoot it down with archers. Sometimes, the daemon comes along. If that's the case, send all your knights at it to keep it away from your archers.
|Beware the daemon.|
If not, just repeat what you did with the fire elemental. The daemon is a lot tougher and might kill a few of your knights and footmen before you can bring him down. But in the end, you'll be left with a passable army against Medivh and his spiders.
7. Medivh, 0-12 spiders
|Medivh is a formidable opponent with a very long range.|
Depending on the situation, Medivh might have a huge swarm of spiders, or none at all. Remember all those footmen that did nothing? They'll run interference by giving Medivh some extra targets to shoot. Your remaining knights and archers should be enough to bring him down. Just make sure your footmen don't accidentally block the door and prevent your knights from getting close and personal. The mission ends when Medivh dies. If it doesn't, it means there's some spider, slime, or skeleton along the way that was missed.