My best advice would be DODGE!!!
https://youtu.be/lsYe9aoY5Vs
against mortars you can try to flank it, but against leigs there really no counter plays since they have so much range and are durable as hell, your best bet would be rush to cromwells and wait until the leig is exposed, then you run in and destroy it.
But in most cases you won't be able to do anything against it. It is just another case of #justokwthings