Asta is a Fire Element Character that follows the Path of Harmony. She provides up to 70% of ATK enhancement through her Charging and a whopping +50 SPD from her ULT, both buffs are applied for the whole team. She is definitely easy to use but her skill requires different enemies to hit in order to fully stack her Charging.
Despite the randomness of her Skill's hit chance, Asta is still regarded as a great support that can be used on F2P Hypercarry and DoT Teams. Her SPD buff also help characters with the Glamoth set to achieve the highest DMG enhancement, although it only takes effect for 2 turns.
Best Cavern Relics
(2) Increases SPD by 6%.
(4) When the wearer uses their Ultimate on an ally, SPD for all allies increases by 12% for 1 turn(s). This effect cannot be stacked.
Messenger is her best in slot Relic as it provides her SPD and enhances the party's SPD after her Ultimate. There are no other relics that suits her playstyle than this one.
Best Planar Ornaments
(2) Increases the wearer's Energy Regeneration Rate by 5%. When the wearer's SPD reaches 120 or higher, the wearer's action is Advanced Forward by 40% immediately upon entering battle.
This enhances Asta's ER by 5% and if she has 120 SPD or higher, Asta has the better chance to go first when the combat starts, making it easier to provide the ATK boost from her skill.
(2) Increases the wearer's Effect RES by 10%. When the wearer's Effect RES is at 30% or higher, all allies' CRIT DMG increases by 10%.
If Asta can reach the required Effect Res, she can then boost her team's CRIT DMG by 10%.
Best Stats

Body
HP% / DEF%

Boots
SPD

Sphere
HP% / DEF%

Rope
Energy Regeneration Rate
Substats
SPD / HP% / DEF%
info
Asta needs nothing more but survivability while in combat. She needs at least 134 SPD to make her turns faster allowing her to restore her Energy for Ultimate uptime.
Best Light Cones
SourceStellar Warp, Light Cone Event Warp, Starlight Exchange, Stellar Convergence
Heir
Increases the wearer's Energy Regeneration Rate by 10~18% and regenerates 1 Skill Point when the wearer uses their Ultimate on an ally. This effect can be triggered once after every 2 uses of the wearer's Ultimate. When the wearer uses their Skill, the next ally taking action (except the wearer) deals 30~50% more DMG for 1 turn(s).This Light Cone helps the uptime of her ULT, making it easier for her to spam it whenever it's needed. The next ally DMG buff is achievable as long as the Main DPS' SPD is 2nd to Asta's.
SourceStellar Warp, Departure Warp, Character Event Warp, Light Cone Event Warp
Cannot Stop It!
When the wearer uses their Ultimate, all allies' actions are Advanced Forward by 16~24%.Dance 3x Advances Forward is a great help for endgame contents whenever it's on dire situation. This gives chance for the entire team to steal the next turn and allow them to prevent any possible outcomes.
Secret
At the start of the battle and whenever the wearer's turn begins, one of the following effects is applied randomly: All allies' ATK increases by 10~20%, all allies' CRIT DMG increases by 12~24%, or all allies' Energy Regeneration Rate increases by 6~12%. The applied effect cannot be identical to the last effect applied, and will replace the previous effect. The applied effect will be removed when the wearer has been knocked down. Effects of the same type cannot be stacked.
The chosen buff from the 3 stat choice are gained randomly at the start of the combat and can't be overwritten unless another characters where the same Light Cone. The said buff will last until the end of the battle.