Kraken Slayer into AS CRIT is really Strong into Bruisers and Squishys.

Build: Berserkers Greaves, Kraken Slayer, Then Rapid Firecannon. At 3rd item your E becomes very deadly to squishy's and even bruisers.

rest of build: Phantom Dancer, Mortal Reminder, Infinity Edge. By 5th item you move very quickly due to 21% movespeed from the crit items, and deal a truckload of damage with 100% crit and huge AS on your E.

Toplane:

Against ranged top laners: This works very well, I had fun running a vayne down with a very high damage E

Against Bruiser top laners: Also works alright, but you have to use R to finish the bruiser off.

Against Tank top laners: Don't build kraken slayer

Jungle: This can work for jungle garen well also

Garen has great clear but not so good ganks. Wait until laners overextend and jump on them with Q and then press E to kill them. R needed vs bruisers.

When to NOT go kraken slayer:

Into tank teams. If TWO or more of these types of champions are on the enemy team, DO NOT GO KRAKEN:

You will not be able to kill them with your E in time, and their full combo of abilities will be able to kill you since you are squishy with this build. Your E has a relatively long 8 second cooldown at level 5, and thats long enough to die since you don't deal much damage to tanks anyways.