The most basic of combos is Justice Punch into Shield of Durand(Must be held down for taunt time and damage to increase) which will then set up for an easy Winds of War Colossal Smash chunk damage. After this you back out and wait for cooldowns to come back up since Galio is as good as a siege minion without abilites up. You don't always have to land Justice Punch to trade sometimes you have to just use it as a gap closer.

After you've used Hero's Entrance on an ally you'll land where your ally was when you first casted it so make sure you use it when they're in the middle of the enemy team or being attacked. It's a great ultimate to counter champions like Diana or Ekko. After landing if the enemy team is near where you landed you're gonna wanna charge up a Shield of Durand for a big taunt. If you manage to hit anyone set up Winds of War into a Justice Punch and a Colossal Smash to finish it off, by then you should get the follow up from either the ally you ulted on or the rest of your team.

When wanting to make a quick combo on melee champs or to make it out of vision in Brush to do big damage you can aim for a Shield of Durand(the cast of W will not be seen in Brush similar to Sion's Decimating Smash. Once you land the taunt that will set up for a good Winds of War and then you can cast your Justice Punch backwards if you're right on top of the enemy to make sure it lands a lot quicker and won't give them as much reaction time and then Colossal Smash to finish it off. If you have runes like Aftershock and Shield Bash it will give you a little extra on your combo. Additionally, you can always use Everfrost since it fits in well at any point in the combination to make sure your champion remains where you want them to be.

Watch out when you cast Galio's E ( Justice Punch)Near walls, this will make the ability possibly clip the wall when you're backed up into the wall making is cancel and go on cooldown, essentially wasting the ability.

Shield Bash gives you additional bonus Armor and Magic Resist when Shield of Durand's passive shield is active, and will keep giving you the additional resistances as long as the shield stay on, Shield Bash's damage has a cooldown of 8 seconds.

Galio's Justice Punch will still go through even if he is affected by most cc abilities if timed correctly, this includes Pick A Card Rocket Grab Pulverize Command: Shockwave among others. Some abilities do interrupt the channel and they include Charm Body Slam

Galio is a champ you pick to have permament mid priority you have full clear backline of caster with Winds of War and Colossal Smash at level 4 if you have an Amplifying Tome, otherwise they will need an extra auto or you can finish them off with Justice Punch's little damage, otherwise wait for level 5 for an additional point in Q.

Once you're level 6 and have Hero's Entrance and a comfortable matchup you should be able to push in your opponent and hover towards bot or top to either look for a ultimate gank (Top laners that are easy to gank with Hero's Entrance include Camille Irelia Darius Wukong Renekton Riven.) These top laners are very easy to gank for and will give you an easier time to do so.

Supports that are easy to gank for (Because realistically you won't look to ult the adc if you're looking for an aggressive gank) include Nautilus Thresh Pyke Seraphine Leona. If your mid laner is missing always ping it and you should ALWAYS have a Control Ward in the river bushes, place it on the side that has an objective alive, if no objective is alive always place it botside. This way you can see if your enemy midlaner is rotating and you can follow up on it.

https://imgur.com/a/s9SYyvR

Thank you for taking the time to read the guide I made and learn more about Galio mid and what works best for him from my experience of playing and my 1.4M points on him. If there's anything you think I can improve upon or typos that I missed please let me know.