The way this Build works is that you'll use the higher Incantation Scaling of Erdtree Seal to cast your spells, while still getting the buff from Giant's Seal by holding it in your off hand. You can even swap to Giant's Seals later on when you get another from another player or in New Game Plus.

You'll use Golden Vow to buff yourself, increasing your spell damage as well as defenses, and you can use Flame, Grant Me Strength for tougher fights and Bosses. Burn, O Flame! is a Dragon killer, and is very effective against enemies you can get under, or that are very large in size, since each column can hit for very good damage.

Flame of the Fell God works well as a setup spell cast before you aggro a tough enemy or at the beginning of a Boss fight when it's closing the distance to you. If you fire off Giantsflame Take Thee immediately after casting this, they will usually detonate together for some serious burst damage.

Giantsflame Take This is your Boss killer and is useful against tough enemies and tightly packed groups. It's expensive so don't use it all the time, just when you need it. The rest of the time use Flame, Fall Upon Them to deal with regular groups of enemies, since it's more Mana efficient and can AoE very effectively, often killing many enemies at once.