So there are 3 steps to easily killing this guy:
1: Wait for them to put down their shield and shoot you

2: While their shield is down walk inside (you might get hit a bit) now you are behind the shield congrats

3: Spam attack in any direction just try not to get pushed out of the shield area as you will have to get back in

You might take some dmg from this however it is a very quick way to kill this guy

You also might be able dash into the shield and through it however i didnt try this lmao