Hi I'm Zahan,
TLDR guide: Kael'thas Patch
Questions: Twitter @addic03 or twitch.tv/addic03
enjoy.

T1: More mana more stuff to throw out or get Frost so you can root more often.

T2: Adds up over time.

T3: Free heals lulz.

T4: Regen and maybe they will hit something every 10 games or so. @20 its going to be frustrating to kill you.

T5: They can't stop the relentless assault.

T6: nom nom nom coil.

T7: Shielding yourself and popping both heals can bring you back from the brink and bait enemies.

T1: Make em popsicles more often.

T2: stab stab stab

T3: Everything else sucks this tier might as well get sustain.

T4: ICE BONE DRAGON BRBRBRBRRAAAAWWRR or you know dead guys if you need to eat them to live.

T5: You don't even need to aim now 360 no scope freeze. Everything in this tier is good.

T6: Stabbing is good healing can be good too but thats not damage.

T7: Allows you to stay in the fight. Sindra 20 upgrade is good too if you aren't worried about living.