Upgrade your equipment to the Mythical stage using the Mythical Enchancement system.
The Enhantment Stones needed for Breakthrough can be acquired by transmuting Premium Transformation Stones and Fusion Stones. Fusion Stones can be acquired as a reward from completing 7 Weekly Challenges, the ongoing Dungeon Challenge, Merchant of Wonders, and achieving high ranks in the Battlegrounds leaderboards.
Mythical weapons can be enchanted up to +20. The earlier levels have a high rate of success of enchanting, but the higher levels will have lower rates of success. When upgrading a Mythical Weapon, the weapon stage can also degrade.
With a Mythical weapon, you can use the Fusion Stones along with other materials to enchant your weapon to increase Boss Attack Power and add Critical Damage at the higher enchantment levels. Mythical status is currently only available to Thornbreaker and Silversteel weapons.