tactics
This is woefully easy, if you do it right.
spell choice
These spells are worth considering:
morph
summon eagle
At the beginning of the level, Gaion will give you the ability to use morph. This is the only spell you will need.
collecting the brimstone
The brimstone you need to collect is in the top left of the map. Select Aurax,
then right click in that general region on the scanner map.
Prees 't' to track Aurax and then select the morph spell - your cursor will change,
awaiting your choice of morph target.
As soon as you see a troll (pasue the game if you need to), right click it, and Aurax will morph into a troll.
In this shape you will be able to walk past other trolls now, without the alarm sounding.
If you see the spell timer start to run out (a yellow/black star object above Aurax's head)
use morph again before you turn back into Aurax.
escaping
Once you find the brimstone, the alram will sound,
and morph wil no longer work (the trolls are suppodly more alert, and will spot your masquerade).
However, despite the claims of Aurax being slow, he will outrun trolls.
Select Aurax, then right click the scanner map to move him back to Gaion.
Press t to track him (the camera will follow) and watch the hilarity, as a line of trolls try to catch him!
He'll take some hits, and on the odd occasion will get stuck and killed,
but in general this approach will work fine.
For an even quicker escape,
summon an eagle,
morph into it,
and fly back to Gaion.
cloak of invisibility
You may have picked up the cloak on invisibility if you've already completed
the village of cissbury level (reurn to).
Don't bother using it in this level - it's valid, but better saved for later levels.
items of interest
in the trees to the south of the bridge is a summon eagle statue