Things you need;
Lots of Ultra Balls
A pokemon Arena Trap
*I reccomend Diglett
Teach it a move with added side affects
*Paralyze, Freeze, or Sleep endusing moves

Get a Pokemon with Arena Trap. Keep that Pokemon in the front of your Party. Now, chase down Mespirit and/or Cresellia, and encounter them. Attack them, with the side affected move. Get their HP at least down to half yellow/orange, and give them the effect. Keep throwing ultra balls until caught