The following list includes a list of, but certainly not all Pokemon that can manually switch in on a Mega-Latios DD and wall/beat the set of DD, Outrage, EQ, HP Fire/TBolt. The move displayed is the move that would do the most possible damage to the Pokemon listed within the set.

+1 252 Atk Mega Latios Outrage vs. 252 HP / 252+ Def Eviolite Porygon2: 136-162 (36.3 - 43.3%)

+1 252 Atk Mega Latios Earthquake vs. 248 HP / 252+ Def Clefable: 144-170 (36.6 - 43.2%)

+1 252 Atk Mega Latios Outrage vs. 252 HP / 252+ Def Mew: 190-225 (47 - 55.6%)

+1 252 Atk Mega Latios Outrage vs. 252 HP / 232+ Def Slowbro: 181-214 (45.9 - 54.3%)

+1 252 Atk Mega Latios Outrage vs. 252 HP / 252+ Def Mega Slowbro: 124-147 (31.4 - 37.3%)

252- Atk Mega Latios Outrage vs. 248 HP / 252+ Def Arcanine: 147-174 (38.3 - 45.4%)

+1 252 Atk Mega Latios Outrage vs. 252 HP / 252+ Def Suicune: 174-205 (43 - 50.7%)

4 SpA Mega Latios Thunderbolt vs. 252 HP / 252+ SpD Suicune: 128-152 (31.6 - 37.6%)

+1 252 Atk Mega Latios Zen Headbutt vs. 252 HP / 252+ Def Aromatisse: 174-205 (42.8 - 50.4%)

+1 252 Atk Mega Latios Earthquake vs. 252 HP / 252+ Def Aromatisse: 145-171 (35.7 - 42.1%)

4 SpA Mega Latios Thunderbolt vs. 248 HP / 252+ SpD Togekiss: 128-152 (34.3 - 40.7%)

+1 252 Atk Mega Latios Outrage vs. 252 HP / 252+ Def Cresselia: 186-220 (41.8 - 49.5%)

+1 252 Atk Mega Latios Earthquake vs. 252 HP / 252 Def Florges: 164-194 (45.5 - 53.8%)

4 SpA Mega Latios Thunderbolt vs. 248 HP / 0 SpD Granbull: 148-175 (38.6 - 45.6%)

+1 252 Atk Mega Latios Earthquake vs. 252 HP / 252+ Def Mega Audino: 101-119 (24.6 - 29%)

A DD Mega-Latios set relying on all physical attacks simply gets hard-walled by essentially all steel types, and a set such as this one that incorporates a special move to compensate for its resists get hard-walled by a large amount of neutral walls even further than just the handful that the list references.
Yes, Mega-Latios does have moves in its FULL arsenal on both ends (physical and special) to hit a variety of things (although variants that conform to either the full physical or full special side still meet its large amount of walls) - it is simply unrealistic to consider that Mixed DD sets are even remotely a severe threat to a league, which consists of so many different drafts.

Sincerely,
Khaz