You may want to use a strategy less reliant on trapping Entei. For example, when I want to catch roaming Pokemon, I use a Pokemon with Thunder Wave to paralyse it, and use a Pokemon with False Swipe in each following battle to dwindle his HP down to one.
Another, similar idea is the Endeavor Strategy. Catch a Pokemon that can use Endeavor, like Swellow or Raticate, and lower their HP down to one. Then, when you find them, just use Endeavor to lower their HP down to one.