Magcargo

Type Tier
Fire / Rock UU
Statistics
Stat
HP
50
303
Atk
50
198
Def
120
338
SpA
80
258
SpD
80
258
Spe
30
158
Name Item

Curser

Leftovers
Moveset
~ Curse
~ Rock Slide
~ Earthquake
~ Fire Blast / Rest

Magcargo needs Curse to even begin thinking about doing some kind of damage—Rock Slide is for STAB, Earthquake is there because it's a good physical attack, and Fire Blast is there so Skarmory doesn't totally laugh at you. However, it may need Rest so that it can actually live long enough to do something useful.

Other Options

"Pathetic" would be the best word to describe Magcargo's movepool. Amnesia might let you live though some special attacks, and Toxic will help to do damage, since Magcargo's attacking stats aren't good at all.

Opinion

Simply put, Magcargo stinks. Its typing is HORRIBLE (4x weak to Ground and Water and 2x weak to Rock and Fighting). Its stats are also bad, as it can't really take physical hits despite the 338 Defense (it can tank some things like Muk, but don't expect it to do much). Its movepool does it no favors either. If Magcargo got Explosion it could be more useful.

Counters

Magcargo has many common weaknesses to capitalize upon; it is also pathetically slow. Even after a few Curses, a good Earthquake will OHKO it, and Rock Slides and Fighting attacks will probably deal a significant amount also. Surf and Hydro Pump are essentially guaranteed to OHKO it, regardless of which Pokémon delivers them. Overall, Magcargo is one of the most easily countered Pokémon in the game, and every team has something that will murder it.