ScarfTran won't like taking an Explosion. It does 90.1% - 106.2%, a guaranteed OHKO with Stealth Rock. Explosion also deals a shit load to Skarmory. Occasionally paralyzing a Pokemon isn't worth losing the ability to soften counters for your other sweepers. For example, if Skarmory wants to set up on you, Explosion will do 54.8% - 64.4%. After a DD, LO Dragonite can deal 38% - 44.9% with Outrage (Life Orb, Adamant, 252 Atk EVs). That's a guaranteed OHKO after one round of Stealth Rock. After you Explode with Metagross, you should switch to something that will force Skarmory out. Explosion is much more useful, even with the risk of doing absolutely nothing.