I am currently on custom rom lineage 14.1 nougat 7.1 and my device is asus ZenFone 2 Z00AD.but i want to come back to stock rom but my stock rom is in lollipop version so, do i need to first downgrade to marshmallow version of stock rom and then to lollipop version ?
You should be able to just go down to the lollipop version. Going to marshmallow then lollipop won't really do anything other than waste time. Just make sure you flash the stock recovery before going back to the stock rom. You should be able to get everything you need from Asus' site.