1) The lich had a terrible set of spells prepared. Really coulda done better.
2) Even with his poor spell choices, he still could've done better with what he had. Open with Stinking Cloud, maneuver while the party can't see him, blast anyone who comes out, cast Poison and then touch whoever looked like the party's mage-type or rogue-type, then proceed to Flame Strike and otherwise blast away at the party's warriors and priests.
As a lich he or she should've been smart enough, wise enough, or Knowledge-able enough to identify on sight which of the party members were probably divine spellcasters (in other words, Reflex-save-bait, so throw Flame Strikes and such at them), which ones were likely the arcane casters or roguish types (aka Touch me and I die!! types, for which the Poison spell and the lich's touch attacks would've been most effective against), and which ones were the melee brutes that he'd have to be wary of (the ones he'd have to use that Ray of Enfeeblement, Rusting Grasp, and Heat Metal against).
3) He should've been wise enough, as a druid and thus Wisdom-based caster, to avoid facing a group of half a dozen adventurers by himself like some kind of fool-idiot. Overconfidence makes no sense for such a character, both as a druid and as a lich. Liches are all about surviving to be 10 billion years old, not stupidly thinking they can survive anything 'just because.'
4) I don't remember if blighters regain their druidic spellcasting or not, but some spontaneous Summon Nature's Ally spells could've helped a bit with the out-numbered thing.
5) The lich should've forced the party to face him on his terms, with a pack of minions/allies at his side as a show of strength and just how bad-arse and commanding he is (even though he's really not, apparently :\ ).
6) For future reference never listen to designers' advice in a module if you think the designer's advice is suspect, or if it doesn't seem to fit the situation of your group specifically.
