Sudowoodo can learn it just lvl up
Rock Slide is not a TM in Leafgreen. In Leafgreen Rock Slide is a Move Tutor move and the Move Tutor is hidden somewhere in Rock Tunnel.
Rock tunnel Uncommon
It is in Rock Tunnel.
Aggron doens't learn rock slide in this version
not, you will need to use tm80, rock slide, on onix.