Wow, congrats.
Killing those Bandits at that level for your first time, must have been tough.
Well what you need to fight Shadowbeast is Strength and Sword, strength is added to your sword damage.
So if a Shadowbeast has say 100 armor, your sword is 50 dam and your strength is 34, then you can hit 84 armor, so it is not level based other than getting more LP to spend.
I like doing everything I can to get as many LP as possible before starting chap 2, ie talking to Lord Hagen whom gives you the key to Valley of the Mines.
You can reach maybe 15 th level before starting chap 2, but that is if you know all the tricks, so shoot for 10th to 12th, maybe.
Whatever you are comfortable with, there are 2 ways to get to the Valley Sneak thru a cave or Fight several Orcs (very tough opponets), which are also the same methods to reach your objective in the Valley.
So you can go very early Sneak or build up levels Fight, Orcs are great XP but very difficult to fight in groups which are easy to run into.
In Gothic it is best to taunt one maybe two at the most away from bigger groups to break down the groups.
To do this you inch forward till one notices you (takes a little practice and works with all creatures and NPCs), then to make sure you do not draw the attention of the others, run back a bit to a safe location to fight, rinse and repeat.