Try skillup before mob takes damage

[ghost]

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Temporary:

• [x] that I understand there are large refactoring efforts going on right now, that these efforts touch every single Lua script and binding, and that my pull request might get put on hold to ensure there are not any conflicts with the refactoring work

In cases where single-hit WS damage is enough to kill a mob, it would be dead before trySkillUp is called (invalid target, causing the function to break out). Moved that call prior to the target taking damage.

Fixes #2577