I'm not exactly sure what you're referring to, as I'm not far enough into the game to know anything about the Leviathan fight, but if you're talking about using AM to get away from an AoE or something, then I'll throw my two bits in. I have yet to figure out the actual dimensions and timings of the various ground circles/cones/columns, because there have been so many times that I have been well out of the area in plenty enough time and still took the damage. I'm still trying to narrow down the circumstances. I thought today that being in the middle of an action somehow affected it, but I'm not totally sure yet.

I know, you're asking specifically about AM, but I wanted to mention that anyway. I don't know much about the AM, so I wouldn't know how to answer, but I can ask questions. How does AM's mapping work? I've never actually seen it in action. Does the character move across the screen or depop & pop? If it's a depop thing, perhaps there's a actually an invisible path being traveled creating a line between depop and pop points and if that line crosses into the AoE then it counts as being in it. That's just my guess off the top of my head. Someone else will probably come in here and have an actual smart answer for you.