Different types of heroes in Heroes 2.

I always take the archer when playing as a Sorceress (as noted, it drops frequently there). I also take the Knight if he has captured a Sorceress castle and replenished his units with his archers (and I often take him in general because I try to have at least two archers in my army – I don’t take them for Necromancers and Warlocks). Magic Shield is very useful when attacking a town, as the towers attack predictably. It saves the archers from total destruction. (There are also options here; if there are few troops in the town, but it can’t be broken in one or two rounds, I cast the shield; if it can be broken in one round, I use magic to help finish off the defenders). I rarely use it in battles on the field (only if the computer attacks predictably).

Added after 10 minutes
Oh yes, and regarding Heroes, I usually take heroes corresponding to the castle, but in general, I prefer Knights and Wizards. I’m not very familiar with the intricacies of Heroes II, so this choice is simply based on my preferences. :)