You should save the game right before you battle it (so if you lose, you can turn off the power and try again), use a move that blocks Mesprit from running, and use weak moves until Mesprit seems like it has 1 HP left (or u could catch a sneasel and teach it false swipe), then use a Poke-ball that seems to be the best for it. (Note:If Mesprit keeps on breaking out of the Poke-ball, then use weaker ones to try and stall it, and then use a good one. Heal Balls are better than they look.)
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You can't, you will need to trade them from Pokemon Diamond, Pearl or Platinum.
You will need to catch the three lake lengendary pokemon. (Mesprit, Uxie, Azelf).
you can go back and catch: -azelf -Uxie -Mesprit and also a SECOND -Palkia and a SECOND -Dialga
entei is a roming Pokemon of Johto. to catch it u have to chase it. if u run or use your bike it will run somewhere else. it is similar to catching Mesprit in Pokemon Diamond,pearl,or platinum.
Yes, you can, but it is confusing, not knowing which is which. It is easier catching Mespirit then attempting to catch Cresselia