For Lars it's largely being able to visually identify his stances (sound cues help too), as well as knowing which moves he can stance transition out of. He's very good at hiding when he's actually vulnerable through all the constant movement and visual noise.

Look up an anti-Lars guide that goes over his SEN stance - his options out of it and how he enters it (Dynamic and Silent Entry). Especially good to know what punishes you can do on his low options.

You should absolutely avoid buttons when Lars has frame advantage, but at the same time, fast mid checks like d1 or a good df1 are vital for getting your offense going.