There is one summoning doll available in each of the 4 acts of the main game. The first one is a reward for helping cure the poisoned imp on the pit level (after jumping down the pit in the arena). You can get back there faster if you just keep going, rather than trying to backtrack. On that level there are a pair of doors controlled by levers, after which you will find a locked room (east of the imps and then part way through the southern corridor)...
<span class='standouttext'>Spoiler : </span><span class='spoiler'>You probably already figured out how to split your characters up to get past the pair of doors. The locked room can be opened by clicking on the torch beside it to extinguish the flame (all the other torches in the room are unlit).</span> (highlight the text to read)
Another technique you can use against strong opponents is to sneak within bow or spell range, attack and then withdraw so they can not find you to counter-attack.
You can also have one character run around to distract opponents (or doing hit and run attacks), while the does most of the damage with a bow or magic. This works very well with melee opponents, but is less effective against archers, and less still with mages that use long range spells, if their attacks can track your character's movement.
Your initial choice of class only determines which skills are available at the beginning, and does not limit how you can develop your characters. You are just about to the citadel, where there are more teachers, so that should help expand your options. You will also find a battlefields key shortly after you get past the mages, though the BF merchants tend to be a little expensive for skills.
