My quick play guide:
Get everyone up to chainmail is step 1, then buy whatever helm and shields you can get. Do all this on sewer fighting. Go town to town fighting in sewers until you can get 8 guys, probably 5 combat (2-3 warriors, 2-3 archers), 2 priest, 1 mage. (mage is optional since they get more useful at high levels, switch them for another combat type maybe)
Put 1 priest on 100% heals, give the other one 1 portal spell and the rest in heals, you will have plenty of healing without spending money.
increase CON 100% for a couple of levels, then switch all stat points to the main class stat. level Tactics on the mage. Level awareness on one priest. level tactics on the other priest. Level Berserker rage on all the combat guys. Get everyone in plate armor, large shield, plate helm. These skills will takes you to level 11, after level 11 you'll have maxxed out their first skill and can be more choosy about a 2nd skill, e.g. train one of the guys in construction.
Check any overland fights and only fight easy battles. Collect gems as you have spare cash: when you see a gem quest, if you have the gem in inventory, you can take the quest, cash in the gem and get free money+XP. Level up on wilderness fights, but avoid ones where it's "tough" or "you've got a shot", or chance is "none whatsoever" as these will probably slow down your leveling even if you win as they will cost you healing or have to revive dead guys. You can do some town quests like "kill 2 groups of monsters" etc around towns. But use the scout ability to pinpoint groups to kill for this as it uses less turns than random wandering (write down the locations).
Switch to dungeon quests as soon as you have all plate/helms/shields, you should aim for artefact quests that have a min. level about 3-5 levels below your party level, so that you can blitz them for XP without needing too much healing.