Forum:Stardust assault vs effect summons

I have so much difficulty understanding Stardust Dragon/Assault Modes effect. He can tribute himself to negate an effect. But can he tribute himself to negate a monster like Darkness Neosphere or Dark Necrofear ? What about cards that activate in the graveyard, such as mystic tomato?24.46.183.194 16:30, May 28, 2010 (UTC)