...Absol? Huh?
Anyways, Gyarados is the easiest water tech, yes. Of course, it's basic is utter failure, but whatever.
If you want a slightly safer alternative, then Octillery has a much more reliable basic (albeit the need for water energies for the first attack), and can do 90+DRE/Scramble discard on Magmortar. You're not really going to be able to keep up with Firestarters though unless you abuse Cessation Crystal like a nut, and then you'll need to figure out the timing on those cards, 'cause Pachirisu will screw you up too.
Magmortar is VERY close to hopeless... Seems like the only solution is a decent power lock to prevent their recovery ability, and a blitz before they get the chance to lay down a single Magmortar. Kinda saddening too, since it actually isn't too bad against other decks :/