Seviper's Attack stat is equal to its Special Attack, and it even gets a strong STAB move to make use of it, making a Choice Band set a great choice. The only downside is that overall a Choice Band set is done slightly better by Arbok, which sports higher Speed and better bulk backed up by Intimidate. Still, Seviper packs more power, and because Seviper has the element of surprise with its massive special movepool, it's still a highly recommended set when considering Seviper for your team.
Sludge Bomb and Earthquake have really good coverage together, with the latter providing super effective hits on enemy Poison-, Steel-, and Rock-types that resist or are immune to Sludge Bomb. Hidden Power Ghost is for Haunter and gets a slightly stronger hit on Chimecho than Sludge Bomb does. Unfortunately, Seviper doesn't get Rock Slide, another perk Arbok sports over its slithery friend, but the last slot here is largely filler anyway. Iron Tail is a good safety move, minus the accuracy, that ensures a super effective hit on Rock-types when using Earthquake could lead to a free switch to a Flying-type or a Levitate user by the opponent. Double-Edge is stronger and more reliable overall than Iron Tail, but this comes at the cost of any super effective coverage.
An Adamant nature and maximum Speed are run to give Seviper as much power as possible while retaining the ability to Speed tie with neutral-natured max Speed Flareon and Chimecho. Jolly is definitely an option, as outspeeding neutral Flareon and Chimecho with this set is heavenly, but the drop in power isn't worth it in the long run. A Naughty nature can be used with a special move, particularly Giga Drain, in the last slot, which gives Seviper a much better tool to use against Whiscash and other physically bulkier Pokemon that resist Sludge Bomb like Relicanth and Graveler.
This set has a hard time with Rock- and Steel-types, even with Earthquake available, making a physical sweeping partner such as Hitmonchan or a special attacker like Seadra or Haunter valued teammates. Defensive Chimecho can be lured in and eliminated, or at the very least sufficiently weakened, with Hidden Power Ghost, further making Hitmonchan and other Pokemon walled by it, like Plusle, good teammates to bring when using Seviper. A switch-in to Flying-types and Haunter is needed for this set to be successful, as they will gladly switch into a Choice-locked Earthquake at the first chance they get, meaning physically defensive Pokemon like Relicanth and specially defensive Pokemon like Kecleon should be paired with Seviper as well. In the same vein, having a good answer to Mawile is crucial to this set's success, as it will use its immunity to Sludge Bomb to set up Substitute or Swords Dance without hesitation, making Flareon a good teammate as well.