Clear out the Temple of Yak, avoiding the encounters with Liches until you're ready. When you are, do the following:
Have your sorcerer/archer cast Lloyd's Beacon and set the location somewhere inside the Temple of Yak. This isn't necessary, but it'll save you some time later.
Leave the temple and head to the Speed Well at E2 (3,4). You'll need mountaineering to do this efficiently (can be purchased in Rivercity).
Walk to F3 (1,7) and toss coins into the Luck Well. Because you cast Lloyd's Beacon and can instantly travel to the temple, you may have time to visit other fountains/shrines/etc. at your discretion. There are quite a few in the vicinity of the Luck Well, so do what you feel is appropriate. Fountain/Well/Shrine buffs wear off at 5:00 am regardless of when you received them, so try to get them as early in the day as possible.
Cast Lloyd's Beacon and return to the temple, where you can now attempt to fight those bastards. Good luck.
The Yak Master is much stronger than the normal Liches, and you'll probably need to kill him in the first or second round of combat to have any hope of winning. He can kill the entire party (or at least most of it) on his first turn. Follow the normal steps as shown above, but grab some of the other shrines in the area before teleporting back to the temple. If you're serious about killing him right now and you want to go tryhard mode:
- Walk up to the Yak Lord's casket in the temple and cast Lloyd's Beacon, setting this location as the home location.
- Leave the temple and walk to Nightshadow and drink from the well (+10 temporary levels) at (7,7) at 5:00 a.m. Save the game. Do not save again until you FINISH these steps. That way you can reload right from the beginning if you make time wasting mistakes during your buff run.
- Use the Nightshadow portal to travel to Vertigo and drink from the well at (14, 17) to restore everyone's health to the new maximum granted by the extra levels. You can also teleport to other locations from here if you know you can get to some shrines near those places in a reasonable amount of time. After that...
- Exit Vertigo through the gate and grab several buffs on your way to the Luck Well (such as +25 hp, +5 armor, etc.), and then do the same thing on the way to the Speed Well. Once you drink from the Speed Well, check the time. If you've plotted your course correctly and taken as few steps as possible between buffs, you should have several hours to spare before 5:00 am. Cast Lloyd's Beacon and you'll appear in front of the Yak Master's casket. Cast the "Day of ___" spells for both Sorcerer and Cleric. Save the game and open the casket. Your party will go first due to the speed buff. Have your spellcasters use their high damage spells (Fiery Flail and Lightning Bolt are very effective). I was able to kill him in the first round before he ever even made a move, but you may need to reload a few times as a result of flukes, misses, etc before you successfully kill him. Save the game. Loot the casket afterwards.
As you're already aware, the Yak Lich has the rather irritating ability to put everyone to sleep with a single spell that it casts every single turn. If everyone in the party falls unconscious, you'll get a game over (regardless of what everyone's actual health is).
Yak Liches have a Speed of 27. Unless at least one party member has a Speed higher than this, the Lich will always get to go first, which immediately puts you at a huge disadvantage. Items and shrines can boost your Speed, and you may want to explore your options for raising at least one person's Speed above 27 so that you can have a better "first strike". Failing that, a higher Luck attribute will raise the likelihood that you will resist the sleep effect. The combination of these two attributes is the key to winning. Regardless, even these things don't guarantee anything, and you'll probably have many frustrating reloads while fighting these jerks.
The low level spell Awaken will wake the entire party up with a single cast, but obviously this won't help if all your spellcasters have been put to sleep. It can turn a loss into a win on occasion, though. Don't forget about it.
Yak Liches have a Physical Resistance of 40 and a "Magic" Resistance of 50. They have no specific elemental resistances, and you may find that using aoe spells is the best way to drop them in the case of fighting more than one at once (ugh!). In any case, they have 80 hitpoints, which means that it's possible to kill them in a single round with characters who are well equipped and optimized. I was killing them at around level 9, but I was using a custom party that I spent a lot of time optimizing at creation -- the default party is a bit weaker and should probably be a little higher level.
One final note on positioning:
There is a Lich behind one of the doors in a room of the temple. If you try to enter the room, it'll shoot you a couple of times with sleep spells before you can even get to it. To avoid this, close the door as soon as you open it and then back up a few steps. The Lich cannot open the door, but it will walk right up to it. Return to the door and open it, step forward and (if everyone's speed is higher than 27) get the first strike. This strategy also works with virtually every enemy in the game, so it's good to make it a regular practice.