You'll find him on the 30th floor of Fiery Field. You unlock that by beating the story portion and getting the hm "Surf", starting the Spinda side-quest. The four dungeons will have Suicune, Raikou, Entei, and Ho-oh.
You have to get to floor 30 and then you battle.
45
entei on fiery field
just keep on fighting it until it asks you to join
first entei then raiku then suicune
Anything.
in firey field
on the 40 floor of mt faraway once you beat rakoi suicune and entei
you fight off groudon and kygore
You beat Entei in Fiery Field. NOTE: You'll need Surf.
entei,suciene+raikou.
Fight him alone with friend bow