Shadow Entei Movesets : Fire spin & Flame thrower
Weather : Windy (no boosts)
It is very easy to duo shadow entei with Two primal groudons. Even lv40 primal groudons would work. Having precipice blades on them will enable you to take it down much faster.
Party used :
Fereish : Lv50 Primal
Chandrabose : Lv40 Primal
Shadow Entei is a good fire type attacker, but there are still better legendaries that outclass it. Still, it would be nice to have one of them in your party for some variety. It does not perform well in the Master league because of all the dragons roaming around. If you are an Entei fan, definitely go get them and possibly even catch a shiny shadow Entei ! Good luck !