Which is the "legend" rank?
It's Master rank.
I think it goes D, C, B, A, AA, Master and each of the letter grades have 3 tiers in them so it goes D1, D2, D3. Someone can correct me on this.
Also the higher you go on the ranks, the more you points you lose on a loss. Earlier on you loss essentially nothing on a loss but later on you can lose up to 70 points at rank B-A.
So are there videos of how to use spell runecraft? I saw karsticles explaining that you have to gain extra turns with the giants and the 18 mana spell, but I don't know how can you setup up this play with spellboosts, since you require to have all this cards on hand, and reducing a 18 mana card onto something playable doesn't sound that easy, but maybe I'm missing something
It's really easy. The entire deck is made up of spells and draw. Even the minions either generate spells, spellboost or are spells themselves.
There's also a card called Sorcery Cache which double spell boosts a single card which means you reduce the card's cost by 3 as Sorcery Cache is a spell itself.
Normally if I get Dimension and the 7/7 in my hand early, I can play them both in the same turn by turn 8/9. The real issue is if you get Dimension in the bottom 3 of your deck or if you get the 7/7 in the bottom of your deck as you need both of them to pull it off. Sometimes you can do some fancy stuff with multiple Dimensions and the few minions in the deck plus the random spell pings to face.
Here's Kripp using it:
https://youtu.be/KjlBplPQEVQ?t=12m57s