If you're set on doing the Flame Bluster (Mag LV.X), you'll have to go with Typholosion to get energy floating on the table ('cause it appears you can't discard DRE to count as 2 fire energy cards). If you just want the SW Magmortar going around, Blissey actually seems like a good partner, due to the two possessing above average HP at all times (Magmar's minimally above average, which is ~50 or 60 HP nowadays), and Blissey benefitting greatly from having a friend spread damage around.
Other then the high HIGH retreat costs, it's not too bad in all honesty. 1-1 Arcanine tech might be needed. Not sure here. (You know, add pluspower, good bye Gallade/Empoleon/plenty of other pokemon)