(Edit) This page has been updated to have the new mastery tree. I'm not sure how effective it will be. Please leave any opinions below. In addition, I've looked over the comments and decided that Frozen Heart might be better than Chalice of harmony in late game. If the Enemy has more AD you might want to consider Frozen Heart before getting FoN (Force of Nature).

Galio is a tank that focuses on lane harassment early game (very effective farmer and good at 2v1 laning). His main purpose in team fights in to help control positioning in your allies favor. He has an aoe slow for the enemies and an aoe speed buff for his allies. these things can also be used for great effect while chasing down enemies or retreating. His ultimate can be devastating when used proper, or be the thing that wipes out your allies. It's a 2 second aoe taunt that deals magic damage to all who's inside of the radius when it reaches two seconds. His ult's power power gets increased for each auto attack landed on Galio during the 2 second duration.

First off, I want to say that I don't remember what level you get the third rank in your ult, but if i made a mistake always grab that whenever (duh).

Q: This is the skill that you want to use for lane harassment. It's very easy to hit someone with it as long as you wait and watch you opponents pattern. Everyone has a pattern. get within range then wait for a second while killing minions. Once you can predict their movement use your Q. If you're good, this should work about 90% of the time. This is also effective for chasing down enemies or allowing you or your allies to escape from combat.

W: This is why Galio is a beast in the lane and just about anywhere. push your enemy back behind his minion line, wait for the next wave of minions, use W and lets them hit you a few times to regain a bit more than a quarter of your hp bar, just don't let them hit you too much. It heals less each time you get hit. If an enemy tries to approach you before the minions reach you, just give em a nice Q burst to send em running. In team fights you normally wana use this right as you initiate (make sure you're getting attacked or it's useless and you'll probably whiff it). you will become practically invincible especially when used with your ult. It shouldn't take to long to CD so your next Bulwark should be on a valuable champion who's getting low on hp. Unfortunately it only heals you, but it does give them a nice defensive bonus which might allow them to kill a few more guys.

E:This skill is a bit lack luster in comparison to Galio's other skills, but it still has nice synergy with his other moves. This will allow u to do a bit more burst damage (not that Galio has that much anyway) and gives a nice speed buff to any allies that walk IN THE SAME DIRECTION THAT YOU CAST IT. Remember to always think of which way you want to move. It doesn't do a bunch of damage so it's USUALLY not very good to just deal damage with it, but to help chase or run away. Don't cast it at an enemy if you're running away unless it will kill them or you know you can kite them.

R: Galio's ult is an AOE ult with a fairly decent aura. Try not to use it on a garen who's just used his Q or a Blitzcrank who's just used his knock up fist skill. this will cause them to attack you and disrupt your ultimate since Garen's Q silences and Blitzcrank will pop You up. You have to be pretty careful about using this skill; however Banshees veil will greatly help you avoid minor mistakes. Don't forget to use it under your own turret if you think you can kill an over confident enemy. Flash into ultimate is usually devastating and will be likely to cause victory if your team has alot of burst and you didn't mess up.

Chalice of Harmony: The reason why I suggest this item first (and everyone else as well) is because it's extremely cheap item, anything with magic resist raises your ap due to Galio's passive and while it doesn't completely solve his mana problem, you'll be able to sit on low, but usable mana levels all during laning phase. You'll probably want to replace this later on

Merc Treads: More magic resist. You should always buy boots. nothing will slow you down for long. Enough said.

Visage: Gives you more mana regeneration and helps your bulwark restore more hp. gives you more magic resist, more hp, and a nice touch of CD which is good for his ult, but also a nice touch for his other skills. It's also a very cheap item, so you'll probably have a great early game with these three items.

Banshees Veil: This is a great item on Galio, but I'm not above substituting it for other items. If they have super hard AD hitters you might want Randuin's Omen. If you do get Banshees, get it because they have alot of AP or interruption on their team. Otherwise you might just want get Omen before Banshees.

Abyssal Scepter:This is pretty much a deathcap for Galio , giving him 93 more AP due to his passive, and lowering all nearby enemy champions resist by 20, and giving him 57 Magic resist. This makes him better in team fights since he can actually do damage now. Not too much more, but enough so he's not completely Ignoreable. This can also help the AP casters on your team.

Randiun's Omen: This will give Galio some more CD (yay), but mainly it helps him tank better. especially against AD heavy teams. If someone like Tryndamere or Nocturne is Atacking you during your ult and you don't have Bulwark up, you'll need some armor to survive. You'll also want to use Bulwark right before using the active on this thing, that way it will slow your enemies down longer (.5 seconds longer for each 100 armor/resist. Galio's Bulwark gives him 90 of each at max level).

(Edited to include my thoughts on Frozen Heart)
I would suggest replacing Chalice with a Frozen Heart, Deathcap, or Morello's tome at the end of this build. If you find that you and your team work together well enough to realize the potential of your ult, then you might want Frozen heart. This will give you a good mana boost, slow your enemy's auto attacks down (great if the other team has a Tryndamere or Yi on their team), and you can't forget about the CD reduction that this has. Though, if you find you might want a little more damage, and are doing fine with your mana management then you might want to consider Deathcap, which can be super useful, upgrading your heal amount, and allowing you to do semi-real damage to people. Morello's is to sustain mana regen, lower CD (always a good thing on Galio), and give a a bit more of an AP boost so you'll almost do noticable damage late game. It's still enough to finish people off when they run.

P.s.: Thanks for all your feedback.