Apparently the typing which I suggested on the previous page has been deleted since I explained too much in terms of the Perfect Partner route (even though I planned to edit it the moment I reached home today since I was on phone Dx ). But then again, the typing I suggested, I believe when explained well, will still fit the bill so I'm giving it another go to explain.
What Pokemon does Mega Gyarados threaten?
Pokemon's without status that can't hit it more than neutral. This seems pretty obvious, but in real practice we have Excadril, Bisharp, Ttar, Heatran, Landorus-T, Mamoswine, Lati@s after Draco Meteor, Starmie without T-bolt, Greninja without Grass Knot
What Pokemon can Mega Gyarados set up on?
The aforesaid list contains mostly of Pokemons who are unable to hit Mega Gyarados for neutral, which it obviously sets up on. In addition to that, Gyarados also finds many common Choiced resistances running around a lot. Common Choiced users that we threaten are Scizor, Ttar locked into Pursuit, Excadril locked into Iron Head, EQ, Garchomp locked into EQ, Landorus locked into EQ, Gothitelle locked into Psychic etc. As for the other key resistances MGyara has, Fire is too risky due to burns, Water is ok if we are sure they lack Scald, Ghosts are not fine unless we are sure it doesn't have Focus Blast/WoW, Ice types usually carry Bolt Beam so it's risky.
Overall, to summarise the list, we have pursuit trappers, scarfed grounds, hazard removers
Factoring in all these, what immediately stands out is a spinblocker, especially one that is Ground weak, which leaves us with Ghost/Electric, Ghost/Steel, Ghost/Fire, Ghost/Rock. Ghost/Steel gives us a weakness to Fire because burns, Ghost/Fire gives us a weakness to Rock, Terrakion in particular, and Ghost/Rock shares a Grass weakness. The problem with sharing weaknesses is that whatever kills CAP19 may not become set up bait for MGyara, which defeats the point of it dying. Hence Ghost/Electric stands out as the better typing of the 4.
Weaknesses: Ground (2x), Dark (2x), Ghost (2x)
Resistances: Flying (2x), Bug (2x), Flying (2x), Steel (2x), Electric (2x)
Immunities: Fighting, Normal
Being weak to Ground and Dark when most users of said moves are set-up bait for MGyara is really handy to have, especially when it gets Pursuit trapped by a choiced user or hit by something locked into Knock Off, effectively giving MGyara a free DD. As for resistances, I find that a resistance to VoltTurn is almost mandatory because KO-ing us with that removes any risk in KOing CAP19. Above all, this typing is immune to Thundurus's Thunder Wave, which all set-up sweepers hate to the bone.
The biggest thing about this typing is how it is a spinblocker, which the opponent does not actually want to KO since Excadril and Starmie are both set-up bait for MGyara. This inherently also supports Bisharp as a partner, whose weaknesses also just happen to be set up bait for MGyara. This basically puts the opponent into a position where they are screwed if they do not remove CAP19, and screwed even more if they do, which is something we want.
In the case that CAP19 can helps something other than MGyara to sweep, Dragonite has no problem setting up on Dark, Ghost and Ground type attackers, and MTtar loves coming in on Dark and Ghost types. (and when it is alive it helps Bisharp sweep)