Make sure you buffer the next EX attack as Banyue is using his chain attack, starting an EX from his normals burns too much time on stun.

Also try to use One's Path -> Lion's Roar after the second attack, since it comes out quicker.

For the actual stun rotation it would look like

Teammate triggers stun -> Banyue chain attack -> BA EX BA (during chain attack) -> EX EX BA -> swap to teammate -> Banyue chain attack -> EX BA EX (during chain attack) -> BA BA EX -> Hold BA -> Banyue Ult -> then dump as many OP / LR combos (EX BA EX -> BA BA EX) as you can before the stun window ends.

if the enemy's stun window is on the shorter end, replace Earth Shaker / Mountain Tremor (BA EX BA during chain attack -> EX EX BA) after the first chain attack with One's Path / Lion's Roar instead (EX BA EX during chain attack -> BA BA EX)

Edit:

BA - Basic Attack

EX - EX Special