Galio is a champion used to counter enemy teams that consist of primarily mages (3 or 4 magic based champions) or to set up AoE oriented teams.
Righteous Gust can be useful for initiating teamfights, chasing runners, or escaping pursuers due to its speed boost.
Galio is a strong pick against AP casters, as he is very resistant to magic-based harass. By zoning with Resolute SmiteGalio and Righteous Gust, casters will become under-farmed and under-leveled, giving your team an advantage.
In accordance with this, try to save Resolute Smite for an enemy that is going to last-hit - the animations that all champions must go through to autoattack forces them to stand still.
Placing Bulwark on an ally under attack is a good way to protect them and regain health.
If you are low on health, you can throw the shield on and draw minion aggro for the duration of the shield. The damage taken will be negligible in comparison to the heal.
Try using Bulwark before casting your spells, as it indirectly increases your ability power via your passive.
Always remember to use Bulwark right before using Idol Of DurandIdol Of Durand, or even using it during Idol of Durand as it doesn't interrupt the channel. The increased armor will minimize damage, and its healing ability may even allow you to end the move with more health than you started with. It will raise ability power as well (as a result of his passive), allowing you to increase your damage output.
Bulwark's lack of casting time allows you to cast it defensively while simultaneously using your other spells offensively.
You will always get the MS boost from Righteous Gust in the moment you cast it for 1 second (direction is initially irrelevant, you will keep the bonus if you keep moving in the ability's path) so you can smartly use it to harass and then avoid incoming counterattack with the MS boost.
You can significantly increase the damage potential of Idol of Durand by using it near high attack speed enemies or when multiple foes are close. Keep in mind that minions are also taunted, and may be used to take more hits with minimal damage.
It can be particularly powerful if Champion's team knows how to use it; the taunt duration can be used get past disruptive tanks, fighters, and to lock down ADC's and APC's.
It's also, notably, one of the few abilities in-game capable of CCing an entire enemy team, so you can use it as a sacrifice, letting your more important team members escape while you take the brunt of the enemy team's damage.
Be mindful of how long you will need to channel Idol of Durand, as the damage reduction does not make you invincible.
Cancelling the channel early after being hit 8 times will deal as much damage as leaving the spell to channel for the full duration.
The CC from the ultimate is sometimes more effective than the damage, so channeling the spell for the full duration in order to maximize the duration of the taunt can be beneficial.
Against enemies with tenacity, you may need to end your ultimate early to ensure they get hit.
Champions with CC reduction include Dr Mundo and Irelia; it would be better to use your ultimate on other champions, or to end it early to maximize its potential.
Idol of Durand can be used to easily get a kill on enemies pushing your turret, as turrets prioritize enemies attacking friendly champions. Pull an enemy in range of the tower and activate it to get the turret to attack them.
Picking up a Thornmail is good against high physical damage teams, and synergizes with Idol of Durand as it forces enemies to take damage from its passive
Randuin's Omen in conjunction with Bulwark's increased armor and magic resistance allows you to slow the enemy team heavily, together with Righteous Gust you are able to catch up with your enemies easily. Keep in mind that the attack speed reduction passive may result in less hits during Idol of Durand.
Banshee's Veil is a very strong item on Galio. It increases his magic resistance, ability power, health and helps with channeling the full duration of Idol of Durand by potentially blocking an interrupt.
Galio excels at tanking spells due to his natural build towards magic resistance; however he still needs to build decent armor unless the enemy team is entirely magic damage based.
Galio lacks powerful initiation (which counterbalances the massive power that Idol of Durand offers). As such, he benefits from items such as Talisman of Ascension that help him get into the center of a fight.
Galio's passive allows him to counter magic damage heavy teams and deal a reasonable amount of damage and heal in return. Consider bringing champions that deal large amount of physical damage that scales in attack damage instead of mages/assassins that deal magic damage. Keep in mind that Galio can also build armor (i.e. Thornmail and Randuin's Omen); and can still punish you with his Idol of Durand.
Galio himself exists to punish teams with too much magic damage, rather than a primary pick. He is utterly hard to kill with magic (both burst or poke) so always have balanced teams between magical and physical damage. Having too much magical damage usually makes Galio appear while a team with too much physical can result in for example Malphite.
Late game, Galio has particular trouble against Veigar. This is because Primordial Burst's will scale off your AP, which means building magic resist won't be as effective at mitigating his magic damage (especially with Void Staff).