Minior picks Meteor or Core Form as it comes in, before entry hazards hit, and checks again at the end of each turn. Hazards that drop it to half HP or less leave it in Meteor Form until the turn ends.
If this Pokémon is a Minior, it changes to its Core forme if it has 1/2 or less of its maximum HP, and changes to Meteor Form if it has more than 1/2 its maximum HP. This check is done on switch-in and at the end of each turn. While in its Meteor Form, it cannot become affected by a non-volatile status condition or Yawn.