Seven’s gameplay is generally focused at one enemy at a time, and it’s the enemy that has the Snakebite debuff. Focus all your attacks on that enemy, and read below (point 5) for switching.
Open with Diving Strike to get your auras & debuff active. This gives you a one-use HP+ called Bondage Whip (detailed further below). You're next time to use Diving Strike will be generally to upkeep your Thoughtful Heart buff unless you want to capitalize on Force Time with a big attack.
Next use Elementalash on the Snakebitten enemy. It'll execute twice and you'll get your attack buff. If a teammate already gave you a generic ATK buff, you can hold off on this and hit point 4 below.
Elementalash should ONLY be used on a target that has her Snakebite debuff. No Snakebite? No Elementalash.
This attack is fire ice thunder element, if the enemy is weak to ANY of these, the weakness damage will override any absorbs/resistances. See our explanations on Elemental Weakness damage in our Glossary for more info.
With your LD buff and ATK buff active, you can use HP+ Bondage Whip to get her remaining buff. Every action by any ally or enemy will trigger it's effect; Seven will have a small AoE attack and a BRV gain each time. Giving Seven battery won't boost the damage, it's locked to her ATK stat.
This follow-up attack scales from FR Bonus but it's not very strong at base (yet).
Consider using her AA, Debuff ATK up before you use your last 2 Bondage Whips!
Sadistic Spikes should be charged now, and it recharges in 2 skill uses or 1 skill and 1 Bondage Whip (you only have 4 Bondage Whip uses though). Sadistic Spikes gives you back a use Snakebite & Lash, and with a 2 skill recharge, Seven can spam skills for over 50 of her turns (just keep in mind she tends to be a fast acting character).
Snakebite & Lash is her often used, limited hit single target attack, which inflicts her debuff. This is the go-to when switching targets, or when enemies cleanse her debuff. This skill recharges whenever you use her EX, so you’ll generally want to use one to help recharge your EX.