Going second, try to get Meltan in there to get some free energy, then get out with an X speed, evolve, and wreck.
Going first, try to rely on Mawile to knock off energy from your opponent.
2 Potions and Blue are uncommon choices, but I think they both pair well with Melmetal's ability. Blue in particular gets melmetal to 160 effective HP, which was very helpful during the Celebi metagame we just went through.
Archiving this deck here, as I will of course be changing it with the new steel types like Dialga EX.