(My first guide, i am honestly super confused on how this works)

With the right setup you will start the game with 77 MR against an ap champion (fiddlesticks excluded) and 69 ap at lvl 1 this will give u a huge advantage, due to infinite poke with Chalice and Doran's, their poke is out of the question, a katarina ult with your W on will deal around 80-110 damage. Poke them out of lane and farm to victory !

Little harder against ad champions, but your W is a beast in this matchup too, getting early lane pressure will reduce their all in potential since a full hp galio with W in hand is unstoppable.

This whole guide relies on expensive runes and galios W, ranked tested 10 times, 80% win ratio as it stands :) (platinum 3)
The pick, and runes will NOT BE APPRECIATED.. EVER, by your teammates.. until they see it in action, there's no feeling as good as using W, getting fully comboed by a katarina and jungle annie (yeah, it's weird i know) and just walking away at lvl 8 with 256 magic resistance. No damage was taken whatsoever.
Even against ad champions midlane, your sustain is really high

Starting dorans ring will grant you 69 ap at lvl one, with 77 magic resistance aswell. Sustain and power, what can go wrong ?

Getting your spirit visage will allow your W to grant some serious sustain, finish with deathcap because it is not as essential as the other items earlier in the game. Against an ad champion the build will grant you less ap, but way better resistance against their ad team, if you get your thornmail and zhonyas, you are UNSTOPPABLE, dealing high dmg in no time isnt needed because you can actually TANK TOWERS, for long enough for your adc to shred both the champions and the tower. With the new changes on the inhibitor turret, this tower will heal you greater than anything else, taking a route near it increases your chance of survival.
Health shredding items and abilities wont affect you, because you will stay on about 2200 hp, but with so high resistances you are basically a 6k hp ulting mundo with your W on. You want zhonyas around the midgame, this will boost your sustain to even greater heights, use this to wait out the cooldown of your W when you are low, and you will be safe to live another day.

Pros :
Can duel almost anyone.
Strong lane.
Incredible start and very reliable throughout the game.
90% chance of an easy laning phase.
No damage taken from any ap champion, whatsoever (fiddlesticks excluded).
Never ever banned, or picked (Old champions have hidden powers).

Cons :
Weak if behind.
Hard to catchup if you lose your lane.
Not as much damage as a normal apc.
Very mana draining in lane.

You want Barrier to maximize your sustain, buying those extra seconds until W comes of cooldown is essential. Flash is always useful, flashing into the middle and ulting will 90% of the time result in a won teamfight.

This build is by far very useful in ranked play, since the burst from the apc doesnt affect you, your advantage is huge against both all-in,combo and poke comps

Farming as galio is rather easy, the more ap you get, the easier it will be for your Q to shred the minions away, i wouldnt recomment taking the raptors (wraiths) since your dps is low early on, and they dont do magic damage so you will lose alot of hp.

Teaming up with assassins is the best for this build, orianna works fine too with your ultimate. If you get in the middle and ult, remember to wait with your W until they actually start hitting you.. AFTER your ult is done, unless a katarina is spinning in your face, always save W until you are focused by their full amount of damage, your ult already reduces 50% so W is not needed, and is definetly wasted there.

Just maximize Magic Resistance, this is the only time you will see and spend ip on magic resist marks, but it's essential.

If you are facing an AD champion, get :
Magic penetration marks
Armor seals
Scaling Magic resist Glyphs
Ability power quintesses

Anything to utilize ap and resistances. This will grant you no real weaknesses, aswell as bringing some extra effort to the team with the Expose Weakness talent. A 15/15/0 seems weird, but its working.

On early levels you want your Q maxed, until both of you reach lvl 6-7 you will put 2 points into W to withstain their increased burst with ult (Unless they are tf, but you wreck tf)

Galio is NEVER.. EVER.. PICKED. He is the Azir of old champs, but stronger.. this is your advantage. I have seen diamonds not knowing why the tower hits are healing me, or why their burst just dissapeared and they died the second after. The strength of your W, granting 90 Armor and Magic Resist at rank 5 is INSANE.

Poke your opponent and dominate lane, against an Ap champion you can just let them hit you, no dmg will be made, and your W will heal it up if the minions are focusing you, after you get your chalice you will have around 100-110 ap, a Q+E combo will not kill all minions, but some precise hitting before will take care of that. Galio is a rather easy champ to farm with, even with only autoattacks, i dont precisely know why, but that's not important :P

Galio is a rarely picked but super strong champion, with this build you will with no problems 1v2 or 1v3 if you are up to time with your build (Good farming, kills etc).
Buying the mr runes will hurt your IP balance hard, since many of the runes will never be used again afterwards. Remember that the longer into the game you get, the more resistant you get to their AD champions, dueling a fed darius at min 30 was no match for me, W on his ult and it will heal you.. it's insane. You can walk away with 100 hp from an adc, W just before her autoattack hits you, and keep walking with even more hp. A 500 dmg zed did no damage to me with a crit from infinity edge, thanks to the ad build with thornmail and W. (it healed for only 13 but thats 513, dmg reduction)

Best of luck, get the stoneman and carry yourself to diamond my friends! :)