Boost Nodes are passives nodes unlocked at level 200 upon finishing the Arcane River questline that improves the Final Damage % of indicated skills. They can also provide bonus effects at level 20 and level 40. 

Each boost node has a main skill and two secondary skills. Nodes that share the same main skill may be used to level each other. You will need multiple Boost Nodes with the same main skill to upgrade the desired Boost Node. Exact duplicates may not be equipped together.

Boost Nodes can be leveled to a max level of 25. Each boosted skill can reach a maximum of level 60. For optimized allocation of nodes in the limited V Matrix, it is recommended that players choose “trios” that offer boosts to the most often used skills. Thus, two trios with different main skills would allow skills shared between them to reach level 50. To obtain the extra 10 levels, matrix slots can be leveled to +5, offering a total of +10 extra levels to the boosted skills.

See below for recommended skills to level by boost nodes from Miea&DC. Main skills indicate skills to level first. Ideally, in the same trio. Secondary skills offer additional damage boosts that are not as significant as main skills. Backups are suggested once the main skills are fully leveled, or as alternatives in trios if the perfect trio is not obtained. Training skills indicate skills mainly used during auto-battle and does not offer significant bonuses while bossing.