I use a Magcargo as a Physical Wall and it's very usefull. But you have to be aware of water and ground type attacks that are relatively easy to predict. And don'y put him against special sweepers.
Role: Defense wall
Magcargo @Air Balloon/Leftovers
Trait: Flame Body
EVs: 252 HP / 252 DEF / 4 SATK
Bold Nature (+Def, -Atk)
-Hidden Power [Rock]/ Ancient Power
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 usefull, 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 usually 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. And you can choose HP 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 HP is very annoying, the third point is that when ancient power secondary effect works it's a great bonus .
Magcargo counters: everything that uses water and ground type attacks. Pokemons that use SATK are something to take care about.
Sorry for the mistakes, it's been a long time since I used my english!