I'd have to concur with the 'phase out' concensus. If you must kill them with Marines, Razorspam with Twin-Linked Lascannons may work. Punching them repeatedly with a Dreadnought should also do it, though getting one close enough may be tricky. A wedge of Vindicators is of course another good option, and could be turned into a flying armoured wedge protecting Rhinos and Razorbacks stuffed with power-weapon wielding Marines or Sternguard to mop up the surviving Necrons afterwards.
Just don't be playing Sisters of Battle and try to kill one. About the only thing in the list that has any realistic chance of killing a Monolith is a Land Raider bought as a transport for an Inquisitor, and that option looks to be about to be removed. That basically just leaves Penitent Engines.