I think the error was that you calculated for single encounters instead of multiple enemies. aoe moves do 75% damage if hitting multiple targets.
https://www.pikalytics.com/calc?attSet=eyJuYW1lIjoiVGVkZGl1cnNhIiwic2V0Ijp7ImFiaWxpdHkiOiIiLCJldnMiOnsiYXQiOjIwMiwiZGYiOjAsImhwIjowLCJzYSI6MjUyLCJzZCI6MCwic3AiOjU2fSwiaXRlbSI6IkNob2ljZSBTcGVjcyIsImxldmVsIjoxMDAsIm1vdmVzIjpbIkVhcnRocXVha2UiLCJIeXBlciBWb2ljZSJdLCJpdnMiOnsiYXRrIjozMSwiZGVmIjozMSwiaHAiOjMxLCJzcGEiOjMxLCJzcGQiOjMxLCJzcGUiOjMxfSwibmF0dXJlIjoiTWlsZCJ9fQ==&defSet=eyJuYW1lIjoiTWFudGluZSIsInNldCI6eyJhYmlsaXR5IjoiIiwiZXZzIjp7ImF0IjowLCJkZiI6MCwiaHAiOjAsInNhIjowLCJzZCI6MCwic3AiOjB9LCJpdGVtIjoiIiwibGV2ZWwiOjQ1LCJtb3ZlcyI6W10sIml2cyI6eyJhdGsiOjMxLCJkZWYiOjMxLCJocCI6MzEsInNwYSI6MzEsInNwZCI6MzEsInNwZSI6MzF9LCJuYXR1cmUiOiJDYWxtIn19
According to this, Teddi with your nature/EV, guaranteed kill on mantine hordes (special defense nature, max iv) would be lvl 45 and below.
Single encounter mantines would be as you said, lvl 53 and below.
Lemme know if this is accurate or if the error was something else entirely.
Great guide! Imma see if I can build a perfect teddi and munchlax now π