If you come in on defog, you'd leaf storm then d pulse. Can you add up the calcs of a unboosted leaf storm and then a + 2 dragon pulse? Asuume speed boosting nature for serp.Yeah I derped there, but my point still stands you're not going to be doing much back with an unboosted, un-STABed Dragon Pulse and you don't actually stop the Defog unless you run Taunt (which you can't do because AssVest).
EDIT:
252 SpA Life Orb Latios Draco Meteor vs. 0 HP / 0 SpD Assault Vest Serperior: 192-227 (65.9 - 78%) -- guaranteed 2HKO
-2 252 SpA Life Orb Latios Draco Meteor vs. 0 HP / 0 SpD Assault Vest Serperior: 97-114 (33.3 - 39.1%) -- guaranteed 3HKO
-2 252 SpA Life Orb Latios Psyshock vs. 0 HP / 0 Def Serperior: 90-107 (30.9 - 36.7%) -- 64.3% chance to 3HKO
Yeah that's not stopping it without significant investment. Min damage is literally 99.2% (96.8% if you used Psyshock for the second attack), and Dragon Pulse can't even guarantee a 2HKO after Rocks without your own LO.
252 SpA Serperior Dragon Pulse vs. 4 HP / 0 SpD Latios: 120-142 (39.7 - 47%) -- 33.6% chance to 2HKO after Stealth Rock
I'd do it but I'm on a phone