Ice Wall and frost breath deserve their own little section on how to use them effectively! These two abilities are what really make Ymir so special(despite being the most popular god when Summer roles around)
Use Ice Wall
Escape(while running, look down, use it, and the ice wall will spawn behind you. Need to have boots for this to work though!)
Prevent ability escapes(escapes like Guan Yu
are prime examples. As the enemy pops his/her escape, spawn an ice wall in quickly, and laugh as the run into a wall for their entire escape)
Save your teammates(this is risky and should only be done when you've mastered Ymir. When your teammate is running away with an enemy in hot pursuit, spawn in that wall between the enemy and your teammate. Use the few seconds you bought for your teammate to save him/her.)
Prevent your enemy from running(spawn in a wall from behind and freeze them)
Slow down your enemies advances(Attacking a tower and need a few extra seconds? Pop in a wall to block the enemy.
Use Frost Breath
-Initiate. Blink in, freeze and let loose
-Poke. This is more for early game as what you need the enemy to do is stand the hell still. Frost Breath
then [glacial strike] to give a little poke. This "scares off" the enemy, forcing them to back off a bit and letting the "****, he can freeze me and deal a bit of damage" be ingrained in their minds.
-Escape: Use frost breath to help you escape(turn around and yell "Freeze!" or help your teammate escape "freeze the enemy and toss a wall in front of them"
-Scare: After a while, your enemy will get used to your frost breath and begin backing off in fear that you'll be casting it on them. Use that fear to your advantage(but don't go overboard with it).