Relations Between Union and States
(1) If a state law on a Concurrent List matter conflicts with a Parliament law (or existing law) on the same matter, the Parliament/existing law prevails and the state law is void to the extent of the conflict. (2) Exception: if the state law was reserved for and received the President's assent, it prevails in that state - though Parliament can still later override it with a new law on the same matter.
This is the key rule for resolving Concurrent List conflicts: normally, a Union law beats a conflicting state law - but a state can get its own conflicting law to prevail within that state specifically by getting it reserved for and assented to by the President, though even then, Parliament retains the last word and can always legislate afresh to override it.
A state can pass a stricter environmental regulation than the corresponding central law and have it prevail within that state specifically, but only if the Governor reserves that state Bill for the President's consideration and the President assents to it - and even then, Parliament could still later pass a new central law that overrides it again.