Best by far is gonna be howling stones. The mage just has to understand the necro is going to be doing most of the heavy lifting.
Start in basement to get the swing of things then go to North. In north, there is a safe spot at corner of the ramp / half wall you can AFK / camp out at as well which is nice. Just be careful of the trap that is right there (you will fall through to basement and likely die if you don't gate out). Treat all carpets as if they are traps to be safe.
Play it like the necro is solo save for the fact you don't have to worry about pets taking XP. Use the charm pets to eat all the harm touches...break / kill low hp necro pets and keep cycling through.
Once you get the hang of it, the levels are going to fly by. You will wipe from time to time and have a blast doing it. Also will encounter soloers and other small groups for opportunities to team up and be social (adding a shaman, enchanter, cleric, 2nd necro, bard, druid...or any combo of the aforementioned will be fun and add more room for error).
Enjoy!
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