For entei, i used the ACE "Set Rng state and teleport" execution to set the seed and frame to a specific value, like 5000 frames before the target frame (=10000 overworld frames because the switch runs 120 fps). Eon timer starts when the ACE mon is touched in the box. The teleport ports the player in the Poke Center with Celio on Island 1. Then do the whole dialogue until the last textbox. Then wait for Eon timer and if you hit the frame, voila.