the main passage seems more like a large north-south tunnel, rather than a vast roundish cave, with the the stream tumbling down on its west side and a climbing path along it, direction north.
across the stream from where Veit is now, on the west-side, there is the smaller tunnel entrance described before (the wolfhouse was on the east side of the main passage, and 10-20' more downstream, closer to the exit.
further ahead, Veit spied the rickety bridge, 20' up above, with the temporarily distracted goblin.
The bridge crosses the main large tunnel and connects two additional smaller tunnels on either side, cut into the sides of the main entrance approx 20' high (the other passage above).
all suggestions seem viable to me:
- magic missile has a high chance to succeed, since it is a certain hit and deals 3d4 damage (all 3 magic missile would be directed at the same goblin)
- sleep is almost certain to work, with its 5d8 roll. it would also target any other creature that might be there but we do not see. however, if none is there and depending on how far the bridge actually is from Master Veit, he could be next, so in that case he should step out of the 20 feet range of the spell effect (he might already be far enough, considering the goblin is high). Syllin is immune to magical sleep so that is not an issue. And syllin, remember that if you use the sleep spell here, you don't lose it, you just use up a slot, you can still use sleep again later on with another slot.
the drawback here is they both use a spell slot, which could be more useful late. That is true, but it is a call that our magikers will have to make over and over again, given the scarcity of spell slots. it's intrinsic to play a magic wielder.
- casting light on the goblin scimitar (or, better, on the bridge rail in front of him, so he there is no check for it) could also work. And since than Master Fletcher would shoot the goblin as a well-lit target, from the dark, the shot could have advantage (up to
@ravenvii). this would save the slot, but the success is less guaranteed, as Fletcher would need to roll at least a 4 on the d8 (+3), to kill the goblin o the spot.
in this case the sequence would be: snuff the light off the sword of Master Fletcher, who quietly moves up to Veit and gets ready to shoot his bow, Veit flashes the light in front of the goblin, Master Fletcher shoots, lights goes off again.
not sure if this can be done stealthily with more than one shooter at range ready to take advantage of the surprise.
@ravenvii , can we coordinate surprise attacks? for example, could Veit and Syllin be next to each other, count down 3-2-1 and unleash their ranged attack cantrips on the goblin which would be hit by both, before having time to react?
if we are to use a spell, i think sleep is a better option than magic missile in this context.
if we have surprise or advantage in our attack, then i think we can get him without consuming the spell slot, although i do not think we can get him alive. we need to kill him from range before he has time to communicate our intrusion.