Thank you for your suggestion. However, I never intended to have Seviper go up against a Ground type Pokemon due to it's type disadvantage. Earthquake is a common move loved by many trainers, and lots of trainers use it on non ground types. Thusfore losing the STAB bonus. Lets use Feraligater as an example:
Feraligater comes out and sees Seviper. Knowing Seviper's frail stats, the trainer is tempted to go for an earthquake which he believes that will OHKO or at least survive with Focus Sash. Upon using Earthquake, it did minor damage with defense bulk, and allowed it to eats it's berry. Seviper retaliates with Giga drain which should deal a butt load of damage and restore it's health. Feraligater now is forced to retreat (assuming it's bulk held up to not get KO'd), which is a good time to stockpile.
There are more than one way to use your Pokemon than just reading and crunching a bunch of numbers. Think about that.
In your example, Feraligatr doesn't need to use Earthquake anymore, mainly because of Sheer Force.
Here's another example: Tyranitar. It doesn't use Earthquake as much, but it's still used on occasion. Not only that, it resists Belch, Dark Pulse and Flamethrower all at once, and takes pathetic damage from Giga Drain, due to the Special Defense buff it gets from sandstorm:
252+ SpA Seviper Giga Drain vs. 0 HP / 0 SpD Tyranitar in Sand: 102-120 (29.9 - 35.1%) -- 15.1% chance to 3HKO
252+ SpA Seviper Giga Drain vs. 248 HP / 224 SpD Tyranitar in Sand: 82-98 (20.3 - 24.3%) -- guaranteed 5HKO
Meanwhile, Tyranitar can do whatever it pleases against Seviper:
Offensive:
252 Atk Tyranitar Earthquake vs. 0 HP / 252 Def Shuca Berry Seviper: 120-142 (41.8 - 49.4%) -- 21.5% chance to 2HKO after sandstorm damage
252 Atk Tyranitar Earthquake vs. 0 HP / 252 Def Seviper: 240-284 (83.6 - 98.9%) -- guaranteed 2HKO after sandstorm damage
252 Atk Tyranitar Stone Edge vs. 0 HP / 252 Def Seviper: 180-213 (62.7 - 74.2%) -- guaranteed 2HKO after sandstorm damage
252 Atk Tyranitar Crunch vs. 0 HP / 252 Def Seviper: 144-171 (50.1 - 59.5%) -- guaranteed 2HKO after sandstorm damage (same applies to Pursuit hitting a switching Seviper)
252+ Atk Choice Band Tyranitar Stone Edge vs. 0 HP / 252 Def Seviper: 297-349 (103.4 - 121.6%) -- guaranteed OHKO
252+ Atk Choice Band Tyranitar Earthquake vs. 0 HP / 252 Def Shuca Berry Seviper: 198-233 (68.9 - 81.1%) -- guaranteed 2HKO after sandstorm damage
252+ Atk Choice Band Tyranitar Earthquake vs. 0 HP / 252 Def Seviper: 396-466 (137.9 - 162.3%) -- guaranteed OHKO
252+ Atk Choice Band Tyranitar Crunch vs. 0 HP / 252 Def Seviper: 237-280 (82.5 - 97.5%) -- guaranteed 2HKO after sandstorm damage
Defensive:
0 Atk Tyranitar Stone Edge vs. 0 HP / 252 Def Seviper: 150-177 (52.2 - 61.6%) -- guaranteed 2HKO after sandstorm damage
0 Atk Tyranitar Earthquake vs. 0 HP / 252 Def Shuca Berry Seviper: 100-118 (34.8 - 41.1%) -- guaranteed 3HKO after sandstorm damage
0 Atk Tyranitar Earthquake vs. 0 HP / 252 Def Seviper: 200-236 (69.6 - 82.2%) -- guaranteed 2HKO after sandstorm damage
0 Atk Tyranitar Crunch vs. 0 HP / 252 Def Seviper: 120-142 (41.8 - 49.4%) -- 23.4% chance to 2HKO after sandstorm damage
And if you still insist on using Belch Seviper, SubSalac is a much safer way to do it. You spam Substitute until the Salac Berry activates, letting Seviper actually outspeed things. Of course, priority and special walls have to be taken out first:
-Substitute
-Belch
-Flamethrower
-Dark Pulse/Knock Off
Nature: Timid/Naive
EVs: 4 Def/252 SAtk/252 Spe (level 50) or 20 HP/236 SAtk/252 Spe (level 100)
Item: Salac Berry