Galio has a really strong laning phase when paired with an aggressive ADC. This will allow him to win lane and snowball his carry.
His Ultimate R is a great team fighting tool that can keep any one of his allies alive for longer. Using it at the right time could also enable him to get a 5 person knockup.
Galio can assist his Jungler, his ADC, or his Mid-laner easily with his Ultimate R. His Ultimate R has such a large range that can allow him to move around the map incredibly quickly.
Galio is a melee champion and can be easily abused by ranged Supports and the enemy ADC. This makes it hard for him to lane against poke champions.
Furthermore, he can easily get interrupted by the enemy if he wants to engage with his E. If he misses it, he will get killed easily too.
In a 2v2 setting, his Ultimate R may not be super valuable compared to other Supports. It doesn’t do a lot of damage like other Supports, and it can be hard to use in the mix of battle.
0 - 15 min
Use the lane brushes to hide and poke the enemy with your Q every chance you get. Staying out of vision will allow you to easily all-in the enemy with your E and W.
Since you're a protective champion, you will want to be the first to respond to fights around the map, especially in the bot side river. For that, ensure that you have the wave slightly pushed up so that you can respond in time.
Once you get your Ultimate R, you should have your eyes glued to the map during skirmishes. Look for champion icons overlapping and see if you can help your allies.
15 - 25 min
During the mid-game, you should be ready to secure as many picks as you can. Work with your Jungler and set up death brushes inside the enemy Jungle.
Chokepoint fights are going to be really beneficial for you during this phase of the game. Just make sure that you can land a multi-man W on the enemy team and then allow your team to collapse on the CC'd enemies.
Try to help your Jungler during neutral, objective fights, especially when the Jungler is in the objective pit. Your Ultimate R will be able to cover the entirety of the pit's circumference, so it will help you disrupt enemies with ease.
Rely on picking off enemies with your E and W in the enemy Jungle. Don't just blatantly all-in without follow-up, though, else you will just end up dying.
Your Ultimate R will be on a very short cooldown now. This simply means that you can save your teammates quite often. You may even use the allied champs who rely on diving the enemy team to secure a massive knockup at the heart of the enemy team.
Making fights unfair for the enemy team should be your primary goal. It is not necessary to kill an enemy off but chunking them out is a viable strat too. So, if you set up a death brush and manage to catch someone, don't over-commit and die instead.
Galio's early game prowess comes from his all-in combo. Hitting level 3 is a significant powerspike. Additionally, he will have both waveclear and a burst damage all-in combo at his disposal.
Getting his boots is a decent powerspike for Galio. This is mainly because he can quickly shove the wave and roam to other lanes with the help of his Ultimate R. It will also allow him to return to the lane quickly.
Galio is one of those champions whose level 6 isn't that big a power spike for himself. This is because it is a team-dependent Ultimate R, and hence Galio should try to compensate by looking to shove the wave in and then roam to his allies.
Galio's Ultimate R will really shine during this phase of the game as he can now impact team fights whenever that ability is up. Additionally, it works really well when his target is the one who must go near the enemies to attack them.
At level 9, his Q will be completely maxed out. This will give a massive boost to his damage and wave clear. In addition, he can now help other laners out frequently and keep his wave shoved in at all times. He will also be able to poke enemies from the lane brushes.
Galio will have his core items at this point. This is beneficial for him, and his team as his overall damage output and survivability will be higher.
Galio should be getting his build completed by now. This will give him a massive spike in his ability to peel for the allied carries. His damage output will be pretty high as well.
Galio's Ultimate R will still be handy during fights due to its knockup and low cooldown. He can quickly go into enemy territory to deward the area/set-up vision and then come back.
Level 16 isn't that big a spike for Galio. Although his Ultimate R cooldown is reduced and the cast range is increased, he won't have any incentive to use it for himself.