It's probably D, but I'm sure it's not E.In all honesty, Bagon sounds like an E. Training a Ralts (Gallade) is a much better idea.
Bagon is encountered in one of the fllors of Metror Falls (one small floor where you can get TM02 (Dragon Claw), the same as in RSE). Unlike in RSE, it's encountered between level 37 and level 40, which is similar to Victory's Road Pokemon. To get a level 40 Bagon, you can do the Repel trick with a level 39 Pokemon.
When Bagon is caught, he will had this moveset:
Bagon @ ???
-Focus Energy/Scary Face
You can use Rock Slide or Fire Balst, moves that comes out from TM.
Bagon evolves to Shelgon t Lv 30 and Shelgon evolves to Salamence at Lv 50. For getting Shelgon, use a Rare Candy and you get a Pokemon with:
65 HP/90 Atk/100 Def/60 SpA/50 SpD/50 Spe
it's not the best, but with those stats is enought o work. it probably still has some Water routes and the entire victory Road to train, although you would have to grind if you want a Salamence for the Elite 4 (it evolves at level 50). I think that the team has to be at least level 50 to defeat the Elite 4 (and if not, you should the Exp- Share during the late battles)* but given how Salamence comes late and the slow ExP group could not be guarantee to be Level 50 during thew Elite 4 and it would require notable extra training.
Given how relatively inefficient it is, how weak Shelgon is (but Shelgon is mediocre it's late-game, it's usable until the Elite 4).
Because all of those details, I would say that at last it's D. E should only be reserved for the Pokemon that (even with Exp- Share) are nearly unusable in normal playthrough conditions.
Gallade should be E completely. Having to deal with a kirlia until level 45-50 (whose movepool doesn't work for Gallade) is useless. If Kirlia is already very few at level 25-30 you will know how weak it is when everything uses fully evolved (and strong) Pokemon.
And using Super Training is inefficient.
1) You have to fully EV one of your Pokemon.
2) You have to enter Super Training and pass all of levels until you have access to this level: "An Opening of Lighting-Quick Attacks!"
3) You havt o defeat that level and you will have a 1/5 chance (is really 1/5?) of getting the Dawn Stone
Too much time to simply get a Dawn Stone. You should stick with Gardevoir.