Undead is always a bit tricky the first time you play them. Be ready, in the heat of battle everyone messes up and forgets and panic heals their undead, killing them. You will kick yourself when you do it, but laugh about it later or maybe laugh when it does happen. After a couple times, you should remember not to. There is an advantage of playing undead. The game will likely attack your undead in ways you hadn't thought of. This is a nice challenge and will help you in future battles against undead.
In a mixed party, feel free to spread the support around. Fane should be able to cover physical armor. Have another character with some points in Hydrosophist for heals and magic armor. It doesn't hurt to have a second character to have a point in Geomancy and Hydrosophist in case the primary character with this gets CCed. In the case of Hydrosophist, Fane makes a good back up for this having spent a point to get Raining Blood. Now you just need someone else to put a point or two in Geomancy to be the back up for that.
Other things to think about are potions and scrolls. Spreading some potions of physical and magic armor around the group can allow characters to take care of their own emergency armor needs. Don't be afraid to use healing (poison, in Fane's case) potions to recover health. Spreading some Fortify and Armor of Frost scrolls around the group helps as if someone does get CCed, another character can use a scroll on them. The four scrolls I like to spread around my party are Fortify, Armor of Frost (these already mentioned), Restoration and Piece of Mind. These four cover the majority of problems that will pop up. OK. I do spread around some Resurrection scrolls as well.
The biggest change when you go to tactician is that opponents will have more health and armor. This will make it a bit harder to CC them. Because they tend to survive a bit longer, some opponents may do things you didn't see during your classic run. They have the same options in both difficulty settings, but on classic they tend to go down quick enough you don't see everything they can do. As such, there are a couple fights that may catch you by surprise. However, that is part of the fun of upping the difficulty. For the most part, that most difficult fights are still the most difficult fights e.g. Bishop Alexander fight at the end of Fort Joy.
The only other advice I would give is that you may wish to delay getting into combat below level 4 unless you have overwhelming numbers, say in the case of Griff's two thugs by Ifan and Elodi when first get into Fort Joy, there you have 6 v 2. Gear will matter a bit more on Tactician, so explore a bit first and should be able to find some gear.
