Last night I figured out how to beat Silke early on. It was fun trying different strategies. Baldur's Gate while not being the best out there in terms of tactics, is probably the best out there in terms of meta imagination. I think the game's main charm.
1. Preparation: Cast Shield before the encounter initiation
2. Denial: Immediately cast Magic Missile (disrupts her casting Improved Invisibility)
3. Saving casualties: Run northeast (until you lose sight of her; she will follow) as if running towards the Temple
4. Orientation: many possibilities; the important thing is to not get caught in ricochet. That's why I run towards map edge, no buildings on the Lightning's way
5. Dodging Lightnings: When she starts to cast Lightning make sure you are in motion. The lightning while being fast is not fast enough and will not hit you if you are moving in orthogonal direction to her bolt (not sure if halflings or gnomes can escape it though as are slow characters)
6. Finisher: She casts two Lightnings and switches to Magic Missile but you cast Shield beforehand so you are immune and feel free to beat her to pulp
Voila 900 EXP Level 1 and Garrick not roasted