First,capture Shadow Lugia.
make sure you have captured most of the Shadow Pokemon on Citidark Isle(you will need some).
Make Shadow Lugia,and Shadow Snorlax your first two Pokemon.
Get a party of nothing,but your highest lvl Shadow Pokemon.
Buy at least 50+ Ultra Balls,you can get the money by battling at Realgam Tower at least 7 times.
Make sure you defeated everyone on Citidark Isle,so you wont waste Ultra Balls on any other Pokemon.
When you fight Greevil,try to capture his Moltres,Articuno,and Zapdos first,because they're his strongest.
Paralizing them,or putting them to sleep is useful,but not necesary.
Use some attacks on them,since you have Shadow Pokemon,no attack will have a risk of taking them out,and same goes vice versa.
When they're in the yellow,or red zone,you can paralize them,put them to sleep,or try your luck by pressing both A and B rapidly,when the Ultra Ball opens,into the claw...if done right,it should automatically capture the Pokemon,if done wrong,the Pokemon will break free,regardless of its status.
Hope this helps!!