15 Sept 2013
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
15 Sept 2013
Neither the cost-reducing ability of Nemesis of Mortals nor the condition of its monstrosity ability can reduce the colored mana requirement of those costs.
15 Sept 2013
Once you declare you're casting Nemesis of Mortals or activating its ability, it's too late for players to try to change the number of creature cards in your graveyard to affect the cost of doing so.
15 Sept 2013
Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
15 Sept 2013
An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.