Galio isn't the most often chosen champion, but he has a versatility in the way you build him that makes him an excellent choice when going against any AP heavy team. Galio can also turn around a teamfight quickly with a well placed ultimate. This guide will teach you how to Galio correctly no matter what role you want to play him. *(roles of ADC Galio and Jungler Galio will not be included in this guide)

Pros:
Great Passive that allows Galio to build almost exclusively magic resist and still be able to do reasonable damage with his abilities.
Excellent at chasing down enemies with the speed boost from his E, and the slow from his Q
Excellent poke capacity when you learn to land his Q
He flies
His ultimate is a large crowd control taunt

Cons:
Galio is not a ranged champion so playing him mid can make farming difficult
Galios' poke is a skill shot that can often be avoided
Galio pushes lanes when he pokes so if you are playing support be sure to ward
Galios' ult can be cancelled by silences or stuns

9 Greater Marks of Magic penetration
9 Greater Seals of Armor
9 Greater Glyphs of Scaling Magic Resist
3 Greater Quintessences of Gold
Enough Said
These runes haven't let me down yet, but like anything feel free to change these runes out for your own personal preference.

Tank:
R>Q>W>E or R>W>Q>E
Caster:
R>Q>W>E
Support:
R>W>Q>E

Generally you may have noticed that your ult should be priority numero uno, and that you don't really need to **** yourself if you don't have your E maxed before level 18

Tank: 0/21/9, being that you want a tank Galio to be tanky it is best to take 21 masteries in defense and only 9 in utility. Those utility masteries will be helpful with keeping you from running out of mana before you get your chalice of harmony.
Caster: 21/9/0, get on my level. If you need me to explain why you should get 21 masteries in offense when you are trying to play AP carry then just trust me on it. The 9 in defense are just to give you the general benefit from the fact they make you harder to kill.
Support: 0/17/13, well how often do you see a mastery page that doesn't get 21 in any of the categories? How about every time you look at this guide that's how often. The reason for this is that a support Galio should still build towards the tanky side but they should also be able to benefit from the income increase and CDR on items from the utility tree.

Heal:
Every good Galio needs heal. EVERY SINGLE ONE! Galio is a generally tanky champion, so sometimes it can be hard to kill him even without a heal, but having heal can make the difference between getting killed in a gank, or tricking an enemy into tower diving to their death. Not to mention after you are level six the usefulness of having heal is even greater. With the huge damage reduction from you ultimate plus the heal it can make the tankiest champions think twice about tower diving you.

Flash/Ghost: either of these spell can be used as an escape mechanism and seeing as Galio comes with a natural speed boost and a slow having an extra escape mechanism can make you impossible to catch.

Exhaust: A good spell to use when you are playing the role of support mostly because of the slow.

Runic Skin: is a wonderful passive for building a tanky or support Galio, or when playing against a AP heavy team. It gives you AP when you build MR, simple as that.

Resolute Smite: Galios' favorite poke ability, after you have your Chalice of Harmony just spam it at the enemy champ it comes with a slow so it is a good idea to combo it with your E to make advancing into auto attack range easy.

Bulwark: This makes you very tanky, and it can be put on any teammate. So whoever is the focus of the enemy fire should have this around them. It also gives you health the more whoever has this skill on them is hit by attacks.

Righteous Gust: Airbending, so call Galio the avatar. This ability is mostly only useful for helping you engage or chase down a running champion. Also since it is much easier to aim than your Q you can use it to pick up some kills on enemy champs who are on extremely low health.

Idol of Durand: Your team fight winning ability. Your best friend. Your most important contribution to the team, this ability can be used after you flash into the enemy team to start a team fight right. It can be used at the end of a team fight to trap your opponents. You can use it when your tower is in being attacked by an enemy champion to make them tank the tower and hopefully die.