According to most sources (example), Slugma is the most frequent event spawn together with Geodude. It has a relatively low catch rate, so why waste Razz Berries and Great/Ultra Balls on it?

Because Magcargo is the most efficient prestiger against Flareon, Arcanine, Charizard and other Fire types with Fire moves. More resilient than Vaporeon and Kingdra (and everything else but the super-slow Chansey and Blissey) and only slightly slower than the much squishier Starmie and Seadra.

Bonus: Magcargo is also very good against Snorlax without Earthquake and it can even be an option against a low-level Blissey.

Stats

Magcargo’s base stats are 139/209/100. The stats ratio is P = sqrt (Def x Sta) / Att = 1.04, which is higher than average (almost the same as Vaporeon). So Magcargo is not very fast, but quite tanky.

Typing

Magcargo is Fire/Rock. That adds to the normal Rock typing (which is a hard counter for Fire, exactly like Water) an additional resistance to Fire moves, that deal therefore only 0.64x damage to Magcargo.

Movesets

Two movesets are good:

Rock Throw / Stone Edge is basically the Rock version of Water Gun / Hydro Pump. This is what we are looking for against Fire.

Rock Throw / Overheat is one of the strongest movesets in the game. This is the ideal moveset for neutral matchups, for example against Snorlax and Blissey.

Performance

According to my CP efficiency calculator, here is the relative performance of Magcargo (in terms of combined time and revive efficiency) against some typical bottom defenders gym sitters:

 DefenderMovesIdeal MovesetPerformance % Benchmark
(best prestiger)
SnorlaxLick / Heavy Slam Rock Throw / Overheat71%Poliwrath
SnorlaxZen Headbutt / Body Slam Rock Throw / Overheat84%Machamp
Flareonany Rock Throw / Stone Edge100%Magcargo
BlisseyZen Headbutt / Dazzling Gleam Rock Throw / Overheat78%Machamp
Dragoniteany Rock Throw / Stone Edge60%Cloyster
Lapras/Cloysteranything but Hydro Pump Rock Throw / Stone Edge or Overheat98%Poliwrath

These rankings are done with a 0%-70%-30% dodge setting, i.e. dodge only specials and more likely to dodge if using a non-Super-Effective fast move (because it doesn’t delay the “Defender used Charge Move” message).

Pokébattler simulation of Magcargo vs. Flareon

 

Limitations

  • Magcargo has a double vulnerability to Water and Ground and a single vulnerability to Fighting and Rock, so don’t use it against Vaporeon or Rhydon/Golem. Also Snorlax with Earthquake can be a big problem, where on the other hand Poliwrath or any Grass prestiger shines against Earthquake Snorlax.
  • Against Blissey, Magcargo takes off a big chunk of her HP in time but there is not much margin for mistakes/glitches.
  • Magcargo’s typical CP at level 30 is 1200, like Raticate’s. Maximum CP at level 30 is 1326. So it’s not very useful if typical gyms in your area start from 2800 CP or more. However in that case you are probably seeing no Flareon and no low-level Blissey.

EDIT: Added Lapras/Cloyster (shoutout to /u/bhutos).