Sp. Attack: 125
Sp. Defense: 95
Basic strategy. OVERHEAT EVERYTHING. If you want to save those precious Overheat PP for something bulkier, you have Fire Blast/Flamethrower (Power vs. reliability, your choice). Ground- and Water-Types are bothering you? No problem, Solarbeam's with you. The last move is up to you: Psychic hits Poison- and Fighting-Types harder, and Thunderbolt hits Flying-Types for massive damage and gives you another option for weaker Water-Types. Or you could use a physical move such as Focus Punch in case they bring out a special wall (Blissey gets hit really hard by FP) and to surprise your opponent, while also making them think twice about sending in Blissey/Audino/Snorlax to sponge your hits.
As you may have already noticed, Magmortar's defense is BAD. That, paired with 2 mostly physical weaknesses and an under-average HP make him an easy KO. A STAB Earthquake from Gliscor or Dugtrio should easily KO Magmortar, and a STAB Stone Edge from, say, Golem or Gigalith, should also do the job with ease. But you have to be careful, that Overheat packs a heavy punch.