Competitive decks are usually structured around a centerpiece Pokemon or particular strategies rather than type combinations. I'll use M Sczior-EX as an example, as it as one of the strongest Metal Pokemon in this format. Its attack Iron Crusher does 120 damage and allows you to discard a a Special Energy from your opponent's Active Pokemon, or a Stadium Card. This attack provides a from of disruption, so a deck featuring M Scizor-EX might be based around disrupting your opponent's play by discarding their resources.
One of the most effective ways to play Lightning Pokemon right now is with Magnezone BKT, whose Ability lets you attach as many Lightning Energy from your hand as you would like. This strategy is effective, but it doesn't really allow for the use of any powerful Metal Pokemon, since you're restricted to unlimited attachment of Lightning Energy. Bronzong PHF was Metal's major accelerator, but it was lost with the recent rotation.
As noted above, Raichu XY is an interesting case. It is known as a tech card because although it is a Lightning type, its attack requires two Colorless energy to use, meaning it can be put into any deck. You could run this with M Scizor-EX or your favorite Metal Pokemon, but keep in mint that it is a Stage 1, so it does require some devotion, and its best utilized if your deck is also running Double Colorless Energy so it can be powered up in one turn.