Build and Runes
Don’t clump together so Galio won’t be able to use Flash and his ultimate to CC your whole team.
Galio can be interrupted during his channeled ultimate, which will free your teammates from being taunted.
Avoid the target that Galio uses Bulwark on, as it will have greater defenses as well as heal Galio they take damage.
Avoid picking champions with DoT against Galio, as this will let him heal even more from his Bulwark. Try to pick AD champions against Galio, as this will force him to build armor and be unable to take advantage of his passive.