One problem with this kind of a matchup is that this is purely based on speculations, as none of Tobias's remaining team is confirmed. I personally don't think Tobias had more than two legendaries, as he seemed really concerned when Pikachu was troubling Latios.
Anyway, I'll post a matchup here:
Suppose Ash won the Kalos League and is now challenging the E4 and Champion Diantha in the Champion's League. How will this play out, assuming he uses GPCISS in all the battles(otherwise he won't stand a chance of course)?
Malva's team- Mega Houndoom, Pyroar, Chandelure, Torkoal, Talonflame, Flareon
Siebold's team- Mega Blastoise, Clawitzer, Gyarados, Starmie, Barbacle, Slowking
Wikstrom's team- Mega Scizor, Klefki, Probopass, Skarmory, Steelix, Aegislash
Drasna's team- Mega Altaria, Hydreigon, Dragalge, Druddigon, Noivern, Kingdra
Diantha's team- Mega Gardevoir, Hawlucha, Tyrantrum, Gourgeist, Aurorus, Goodra
How far can Ash proceed? How will Ash's battles against each of the E4 members and Diantha play out? Discuss.(One thing is we can estimate that E4 ace>Champion non-ace>E4 non ace- this is evident from Cynthia vs Flint, as Cynthia required Garchomp, her ace to beat Flint's ace, Infernape).
Since you asked I'll give you my hypothetical play outs.
Ash vs Malva:
Pikachu vs Flareon -> Pikachu
Pikachu vs Torkoal -> Torkoal
Snorlax vs Torkoal -> Snorlax
Snorlax vs Taloneflame -> Taloneflame
Charizard vs Taloneflame -> Charizard
Charizard vs Pyroar -> Pyroar
Infernape vs Pyroar -> Blaze Infernape
Blaze Infernape vs Chandelure -> Chandelure
Sceptile vs Chandelure -> Draw
Ash-Greninja vs Mega Houndoom -> Ash-Greninja
Ash vs Siebold:
Sceptile vs Clawitzer -> Sceptile
Sceptile vs Gyarados -> Gyarados
Pikachu vs Gyarados -> Pikachu
Pikachu vs Barbacle -> Barbacle
Infernape vs Barbacle -> Blaze Infernape
Blaze Infernape vs Slowking -> Slowking
Snorlax vs Slowking -> Draw
Charizard vs Starmie -> Charizard
Charizard vs Mega Blastoise -> Mega Blastoise
Ash-Greninja vs Mega Blastoise -> Ash-Greninja
Ash vs Drasna:
Charizard vs Noivern -> Charizard
Charizard vs Kingdra -> Kingdra
Snorlax vs Kingdra -> Snorlax
Snorlax vs Druddigon -> Druddigon
Sceptile vs Druddigon -> Draw
Infernape vs Hydreigon -> Blaze Infernape
Blaze Infernape vs Dragalge -> Dragalge
Ash-Greninja vs Dragalge -> Ash-Greninja
Ash-Greninja vs Mega Altaria -> Mega Altaria
Pikachu vs Mega Altaria -> Pikachu
Ash vs Wikstrom:
Infernape vs Steelix -> Blaze Infernape
Blaze Infernape vs Skarmory -> Skarmory
Pikachu vs Skarmory -> Pikachu
Pikachu vs Aegeslash -> Draw
Sceptile vs Probopass -> Draw
Ash-Greninja vs Klefki -> Ash-Greninja
Ash-Greninja vs Mega Scizor -> Mega Scizor
Charizard vs Mega Scizor -> Charizard/Draw
Ash vs Diantha:
Pikachu vs Hawlucha -> Pikachu
Pikachu vs Gourgeist -> Gourgeist
Charizard vs Gourgeist -> Charizard
Charizard vs Tyrantrum -> Tyrantrum
Sceptile vs Tyrantrum -> Draw
Infernape vs Aurorus -> Draw
Snorlax vs Goodra -> Goodra
Ash-Greninja vs Goodra -> Ash-Greninja
Ash-Greninja vs Mega Gardevoir -> Mega Gardevoir
Ash vs Diantha: Mid 6-5, Ash vs Wulfric: High 4-6/Low 5-6, Ash vs Other Kalos E4: High 5-6
Interesting so GPICSS matches up best against Wulfric.