In certain areas there will be 2 Big Daddy's at one time, one will have a Little Sister and one won't.

1) Use the hypnotize Big Daddy Plasmid on the one that does not have a Little Sister.

2) Attack the Big Daddy with the Little Sister and the other Big Daddy will start to protect you by attacking.

3) Watch the health of both Big Daddy's and when they get close to dying start using your frag grenades to finish both of them off.