IMPORTANT TIP: There are 2 ways to use E.

1. Tap E to instantly slash in front of her dealing electro damage.

2. Hold E to put up a shield that absorbs damage and release to dish that damage + more back to the enemy.

Using E will apply electro proc on yourself, so just be careful not to take hydro damage or you will stun lock yourself.

PSA #3:

Her E has some explaining to do:

1. The shield from holding E scales with HP. The more HP = more you can store damage (but you don't need to build HP).
2. The SHIELD WILL BREAK IF THE DAMAGE ABSORBED EXCEEDS IT'S TOTAL HEALTH. This will forcibly release the E and you WILL TAKE THE EXCESS DAMAGE.
3. Taking 2 instances of damage while holding E will grant it maximum damage (caps at 2 stacks).

Example: no attacks blocked = 200%, one attack blocked = 400%, two attacks blocked = 800%

Retribution

Counterattacking with Tidecaller at the precise moment when the character is hit grants the maximum DMG Bonus.

Basically you need to learn how to use her E to dish maximum damage. It isn't strict, so you can still get perfect counters if you do it a second too early or late. YOU CAN TELL IT'S A PERFECT COUNTER BECAUSE A GIANT ELECTRO AURA WILL OCCUR.

Lightning Storm

Gain the following effects for 10s after unleashing Tidecaller with its maximum DMG Bonus:

•     DMG dealt by Normal and Charged Attacks is increased by 15%. ATK SPD of Normal and Charged Attacks is increased by 15%.
•     Greatly reduced delay before unleashing Charged Attacks.

Getting a perfect counter or taking 2 instances of damage while holding E procs this passive. Makes Beidou slap like an actual truck. My advice is getting good at perfect countering arrows from Hilchurls and you'll be a master.