If you go to where Zevlor is standing here in the grove there is an exit to the left up some rocks. When you go out this exit Karlach's spawn point is a jump down the cliff on the right. You'll take like 90% damage if you don't have featherfall or something similar like misty step but you get to her super early and don't have to deal with any other combat.
I'll do anything to get to my favorite companion early.
Video link demonstrating the route to her:
Ah that character is a companion? I think we just murdered Karlach because some guys in a house told us to…and then we murdered those people as well because one of us chose an option that makes them attack you.
Next run then maybe…
Oh man you’re in for a treat on your second run. Karlach is easily the best companion with the most personality.
You don't save scum? Just me, then...
Yeah, but I can't play Baldurs gate without save scumming I'm too weak.
https://media.tenor.com/03W6Au3O7DwAAAAM/strangle-bingus.gif
Sneak kill Karlach - Wyll gets paid (long rest) Revivify Karlach, recruit, and you’re all friends!