What's the best way to catch entai?

The only thing I can think of to do is to keep trying. Try and use many different kinds of Pokeballs. Just keep trying is the main thing though. You'll get it eventually.

This tactic has allowed me to catch many roaming pokemon. This tactic requires at least
1. 30 ultra balls
2. 5 fast balls
3. 5 level balls (optional. use only if pokemon level is lower than yours)
4. 10 timer balls
5. 10 great balls
6. a pokemon with false swipes
7. a pokemon with a trapping move or ability

first you have to trap it with mean look or a pokemon with the ability sand trap. then reduce it to yellow. (make sure yur pokemon has a move that does not do very much damage). finally use either an ultra ball, fast ball, great ball, or a level ball.
idf that doesnt work, reduce its health a bit and use the same ball, once it hits red, start using false swipes. then use a ball. finally, after 30 turns, start using the timer balls too.
if any ball shakes 3 times, try the same ball, then go onto a different ball if it goes back to 2 or less shakes.