Yeah, sorry about that. I got an email about your reply yesterday, which was weird since you replied almost a week ago. It seems like the reply alerts are being a bit spotty.
There should be an invisible rectangular box that encompasses both of her thighs so if you're having trouble you can 'cheat' it a bit by clicking between her legs. Since random chance is involved though it is entirely possible you could go through a few battles without the weak spot happening to land on that spot too.
I'm going to address the elephant in the room here in detail (but please read my main comment about this game too). I think these elf fights are too cryptic, which makes them harder than they should be. Many of the hitboxes for the elves are too small and hard to find, and the way the fights work are not explained well. All that Helen tells you is that you need pearls and that the elves have sensitive spots, but there's nothing to tell the player that the sensitive spot changes (which is pretty important to know) or that there's only one at a time, or to hint at what they are. You have to think about it from the perspective of a player with no prior knowledge on this. They're expected to find the one small sensitive spot, figure out that they're supposed to click on it repeatedly, that it then changes to another small spot that they have to find, and guess whether they have enough pearls or just aren't doing the fights right? All through trial and error? That's expecting a lot, especially with how big a shift that is from regular, mindless clicking.
Helen should be more clear with how the fights work such as saying something like "sensitive spot that changes" and preferably add: "shortly after you find it". You may even consider telling the player a recommended amount (or range) of pearls, either for each fight or just enough for the hardest one, so that players do not have to keep resetting their progress just to beat them.
The sensitive spots should be easier to find. The best option is probably to make the hitboxes bigger, so that
clicking anywhere on the body part that the sensitive spot is on counts (which is more expected), rather than just a smaller part of it. This might sound too easy, but it wouldn't be really. Players would still have to find the one sensitive body part, it'd just be more fair. For example the whole thigh (not just inner or outer), belly (not just the belly button), face (not just a cheek), upper arm (not just a shoulder or elbow) or lower arm (not just an armpit). Breast and ear hitboxes aren't as bad but they still may not cover the whole breast or ear, and they should. In addition, if there are two of the same body
part such as the ears, shoulders, arms, breasts or thighs, maybe clicking
on either one should suffice. I mean, how much sense does it make for one to be sensitive while the other is not? Besides, who actually enjoys having to click on both of all these parts, or clicking on one and then later
finding out that you had to click on the other one? Another option is
to have the hitboxes be outright visible to the player (instead of
making them bigger) or provide an option for the player to make them visible. Not necessarily which one is currently vulnerable, just the
hitboxes themselves.
Maybe not what you were expecting, but there's a guide for the boss weakspots here https://steamcommunity.com/sharedfiles/filedetails/?id=1868890259 It might not be perfect, but I think it shows the general areas of the weakspots. There's also more complete guides for the game there that include the bosses in one of their sections.