I use a Magcargo as a Physical Wall and it's very useful. You have to be aware of Water and Ground type attacks that are relatively easy to predict. And don't put him against special sweepers.
Role: Defense wall
Magcargo @ Air Balloon / Leftovers
Ability: Flame Body
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Lava Plume
- Recover
- Hidden Power [Rock] / Ancient Power
- Toxic / Will-O-Wisp
Easy. Put Magcargo against Physical offense Pokemons and see that it can be very good. You can choose into Toxic, WOW for the first slot, if you want to counter walls Toxic is more useful, and Lava Plume has a good chance of burning. I prefer WOW, but just because I want to make sure that Magcargo will burn if it needs. But I usually don't need to use it, because Flame Body and Lava Plume take care of it. Recover is a very reliable recovery move, so after burning or poisoning (Making the physical adversary even more weak against Magcargo) you can Recover and make a big problem to the defensive Pokemon. Lava Plume a good STAB move. You can choose Hidden Power [Rock] or Ancient Power. I prefer Ancient Power, because you don't need to attack to much with Magcargo, so the lack of PP is not a big problem, the second reason is breeding with Hidden Power is very annoying, the third point is that when Ancient Power's secondary effect works, it's a great bonus.
Magcargo counters: everything that uses Water and Ground type attacks. Pokemon that use Special Attack are something to take care about.