Most shrouds don't wait for a Bard to fill anymore, so definitely expect to cover those spells. Displacement and Blur generally don't get turned down by anyone, and remember to Haste the party as they beat down portals. If you toss the Haste before they ask, brownie points will be awarded.
Be as self-sufficient as possible, ideally. Casters in Shroud tend to be moving around a lot, and the healers' main priority will not be on you. You're going to be out of range, or blocked quite a bit of the time. So make sure to keep things on you to keep yourself alive, whether it be pots, scrolls, reconstruct, whatever.
Breaking the crystal in part 2 is the big one. Stack firewalls on it, the more the merrier. Once the bosses start dying, you won't have much time, so stay alert. But this is really the only place I've seen firewall used; it's not really effective anywhere else.
It's always helpful for the casters to take over some of the buffing duties from the other classes; same with rangers and paladins. Toss what you can on the party; it'll be appreciated.
Dancing ball can cause horrific lag in part 4; while effective, make sure you make it go away if you can, especially before Harrry drops. The party will thank you for it.