Lead the Small Knight with the shield, next to the Big Knight with the shield.
Their shields drop when they are close together. Then you focus your fire on the BIG Knight, not the SMALL Knight...
DO NOT--And I cannot stress this enough--KEEP LEADING THE SMALL KNIGHT AWAY FROM THE BIG KNIGHT...
DO YOU FRICKING GET IT??
STOP. LEADING. THE. SMALL. KNIGHT. AWAY.
SMALL. KNIGHT. NEED. TO. BE. NEAR. BIG. KNIGHT.
STOP DOING IT... STOP IT.
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Even when people follow this they just get yeeted away by the boss stomp and the knight follows them. Pretty counter-intuitive for a boss that requires someone be close to him.