When Galio charges towards you, try to disengage as quickly as possible. If you let him get close, he will look to engage with his E and then taunt you with his W.
As soon as Galio goes missing post level 6, ping your team as he may be looking to roam with his Ultimate R.
If you’re a ranged champion, use your range advantage to harass and poke him down whenever he moves forward to last hit.
Avoid grouping closely together in team fights as it may enable Galio to get a 5 man knock-up with his R once one of his allies has engaged. Space out between one another for maximum survivability from the get-go.
If Galio uses his Ultimate R when you’re near him, you can disrupt it with any crowd control abilities. This will be very helpful as it can stop him from using his Ultimate R to help his allies.
In team fights, try to harass and poke him down as much as possible before he can look to engage. By poking him down first, you’ll make it impossible for him to look for an engage with his E and W.
Try and stay at max range at all times to make it very hard for Galio to use his E and W aggressively to taunt you. He is much stronger and has good gank set up when he’s healthy and both of these abilities are up.
Galio’s biggest power spike is reached at level 6. His R gives a large opportunity to gank and assist other lines or introduce himself in a team fight. However, he doesn’t have much kill threat against enemy laners in a 1v1 at level 6.
After Galio has completed his first item and has many points in his Q and E, he will be able to quickly push the wave and roam. Try and match his wave clear and keep him pushed in so he cannot roam for free.