system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher] In Fighting Against Striga, Why Geralt Didn’t Just Lock Himself In The Tomb The Whole Night? He Could Avoid So Much Hassle.
gpt
A couple of reasons. For starters, he had to establish that the curse was reversible, aka he was dealing with a typical Striga.

Through the fight, he tests the monsters sensitivity to silver, a trade unique to monsters whose existence is based in some way on magic to confirm that it is indeed an active curse that is keeping her transformed and it can be broken. How sensitive to silver the Striga was, was also an indicator of how strong the spell is (does he need to stay in one night or 3 nights for example).

Then there is the human cost aspect, while Geralt isn't the sentimental type he does like to avoid unnecessary death. By fighting the Striga first she wold agro on him and at least for the first night stay in the castle trying to get back into the crypt.

Furthermore, the Yrden Sign isn't all powerfull so it has its limits. The shorter it would need to bare the Striga from entering the crypt or moving the sarcophagus lid the better, the weaker from a previous fight the Striga was that much better too. Also, the sleeping potion Geralt took had a specific time-window to work, he had to wait to be inside that window before he drunk it.