I trained mine up a few weeks ago and after trying different spells at different skill levels, this was the optimum way I found (give or take a few points, as I didn't write a log so I'm going off memory here). Works best if you have a soulstone to skill mule it:
First, get resist spells up (I'll explain why further down); I had mine at 92ish, which takes an hour or so to get in Old Haven. Your suit should have 100% LRC, 40% LMC, and then any mana regen you can fit in. FC and FCR helps, but is the others are pretty much a must-have. Boomstick and pendant of the magi will go a long way here. Make sure your int is 125 for training. Next, buy/create/newbie quest necromancy and spirit speak up to 50. Why spirit speak? To summon a shadow wisp to help with mana regen in training, and also to heal damage on yourself later from lich form. Now for the training:
50-65ish: Horrific Beast Form
65ish-75ish: Poison Strike (Note: get into Lich form at 70 and stay in it for now, the mana regen will help. Just throw in a spirit speak every now and then to heal up.)
75ish-90ish: Wither
90ish-100: Lich Form
This allows you to train the skill in the comfort of your own home and not have to worry about chasing down monsters to kill for training. The slowest gain area I found was around 85-90, as wither wasn't giving gains very often but lich form still failed too often to be worth it for training. But by the time I hit 90, I had 100 focus/meditation and since I was going in and out of lich form, I just set my macro key up to cast 5 lich forms and then a spirit speak and rarely had to pause to regen mana; the shadow wisp helps a lot too, since you should have VERY low karma by this point. One key training at its finest.