I'm thinking that Entei Suicune Legend has potential as a Pseudo Garchomp C LvX alongside Feraligatr Prime. Dump Four Water energy with 'Gatr, Snipe for 80 and return two Waters to your hand, rinse and repeat.
With a Standard 'Gatr deck, your already running copious energy, so Legend Box has a high chance of picking up a few energy after using it.
With a bit of luck, interviewer's questions, rare candy and warp energy you can easily set up a donk with 'gatr and ESL T1 (if you go second), or get a quick setup and consistent damage output for the rest of the game.
The same thing can be done with KGL, to cripple your opponent's bench. Feraligatr Prime is probably the best Legend support engine around, in fact.
Ho-oh Legend can be used with Meganium Prime and Sceptile SF to set up late game KOs, as long as you strip dying pokemon of Energy before they go down.
Celebi Prime is also a useful addition to combat their energy costs, using grass as the colourless energies and attaching the required colours manually, will have the Legend ready to use within two turns, without having to rely on lucky draws from Legend Box.