Galio is a very stong team player, and solo mid laner. here are some tips on when to pick him.

.Using his kit to your advantage is key to winning any match up, and for Galio, you have to choose wisely when to pick him.

.NEVER BLIND PICK GALIO!
(he will simply be countered by an AD laner or a heavy ad team, you must use Galio as a counter pick, and unfortunately, hope the enemy team picks the right champs for you to counter)

.PICK INTO AN AP LANE
(this is due to his great passive, which converts MR into AP, making him great against a heavy AP lane)

.ONLY ONE AP ON ENEMY TEAM?
(most people think you can only pick Galio into a full AP team, but this isnt true, as long as your match up is AP, it shouldn't matter how much ad the rest of their team possess)

.AP TOP, MID AND JUNGLE ON ENEMY TEAM?
(this can both be good and bad for Galio, on the plus side you can build plenty of MR, and become indestructible to the enemy, whilst still dealing damage. However, due to the way his ultimate works, it can be a problem) - more on that in next section -

Remember Galio is an AP mage caster like any other, but you have to play him a little different in team fights.

.POKE
(The main thing to do with Galio is to poke, his abilities provide a ton of poke and damage, just make sure you watch your mana bar!)

.POSITIONING
(As Galio, your want to be in the front line of your team, using your abilities to poke them off, make sure you keep the enemy team from engaging by scaring them off with your Q slow)

.USING GALIO'S ULTIMATE
(Galio's ultimate is key in every team fight, mess this up and your in trouble! the key is to stay at the front line looking for the perfect opportunity to engage with your ult. A good tip is to save flash to use with your ult, and catch the enemy off guard. Make sure you catch as many people in your ult as posible, and dont head for the back line, the idea is to make most of their team focus you, whilst keeping the enemy off your AD carry and other damaging champs so that they can beat down on the enemy whilst you have their attention. Try to get their AD carry in the ult too, this means you will do more damage once it goes off)

One of Galio's most useful skill, is his counter to champions with a dot/burn or drain ability.
Galio has a very unique shield, other than increasing the targets magic resist and armor, it provides Galio with a heal, each time the unit/Galio suffers damage while wearing the sheild.
This means champions such as malzahar, swain, twitch (the list goes on, see below for full list) who all have damage over time ability's, are countered by Galios shield, as instead of damaging him, he will be healed instead. (any DOT in the game, such as ignite, can be countered by his shield)

Use Galio's shield when anticipating lots of poke, or when suffering from the damage over time effect.

.List of champs with DOT abilities.

Brand's Blaze
Caitlyn's Yordle Snap Trap
Cassiopeia's Noxious Blast
Darius's Hemorrhage
Fizz's Seastone Trident
Gangplank's Grog Soaked Blade
Malzahar's Malefic Visions
Mordekaiser's Children of the Grave
Nautilus's Titan's Wrath applies to all units in an area around his attack target.
Nidalee's Bushwhack
Singed's Poison Trail
Swain's Torment
Talon's Noxian Diplomacy
Teemo's Toxic Shot and Noxious Trap
Tristana's Explosive Shot
Trundle's Subjugate
Twitch's Deadly Venom
Udyr's Tiger Stance
Urgot's Noxian Corrosive Charge
Viktor's Death Ray if Augment: Death is purchased.