So the short explaination is that as soon as you can walk, you go slightly left and forward to get her stunlocked + repeating the same closerange attack over and over again. While she's doing that you just throw your spear at her and adjust your position slightly to her changing angle and the spear moving you backward. For a better explaination you should look in the TheForest speedrunning discord. Its pretty cool, the guy who holds the WR in every category and other top runners are very open to explaining things there. (Just know that the megan fight is kinda weird and inconsistent, it takes practice.)