Name: Seviper
Low-Tier
Availability: Can be gotten right before Skyla by going north of Mistralton City. Found at a 5% encounter rate.
Stats: Good offenses, but everything else is poor. Notably, this means Seviper has to take a lot of hits due to its speed that it can't handle well with its defenses.
Typing: Pure-Poison is a subpar typing, both offensively and defensively. It hits very few things (and virtually nothing in major battles) super-effective. Defensively, it's nice for fighting types, though neither the bug or grass resistances help that much lategame. The poison resistance is nice in theory, but Seviper doesn't fare well versus most other Poisons due to its lack of secondary STAB or good stats. Its Psychic/Ground weaknesses are somewhat common as well.
Movepool: Seviper's only saving grace, to an extent. Unfortunately this does not apply when you first catch it, where its strongest move is a STAB Venoshock. It has Night Slash for limited coverage, but Ghosts are not that common and Psychics are generally faster than Seviper is. It won't get a better STAB until level 42 where it gets Poison Jab, which helps a lot, followed by a buff to Night Slash courtesy of Crunch at level 45. Seviper's most notable movepool addition comes late- it gets Coil at level 49 (though much to my dismay, Seviper doesn't learn Gunk Shot), which is its best bet for being useful versus the Elite 4. It also gets Flamethrower and Giga Drain, but Flamethrower is exceptionally late (not until after Team Plasma is finished) and you will need to grind a few shards to get Giga Drain (as you won't get enough just going through the game).
Major Battles:
-Skyla: Doesn't fare well versus anything, really. The best you can hope for is to Glare her Swanna.
-Drayden: Again, does nothing of note except Glaring something (which includes Flygon- its Earth Power is pathetic, my Seviper had a -Sdef nature and still survived with ~20 HP).
-Marlon: If you get Giga Drain, it would probably do pretty well here. Marlon's team is very slow, though Carracosta could potentially beat you with Sturdy and Shell Smash. If you don't have Giga Drain, Seviper will probably just get off a Poison Jab or two before dying.
-Colress: Unfortunately, you don't have access to Flamethrower until after all the Plasma business is done, so Seviper goes from being potentially decent to awful here. You can Crunch his Beheeyem before dying, I suppose.
-Ghetsis: Seviper loses 1 on 1 versus his entire team. It can do some good damage to his Eelektross or paralyze his Hydreigon (if you don't get flinched by Dragon Rush [or even possibly KOd!]), at the very least.
-Hugh: If you didn't pick Snivy, then he will have a Grass type-Simisage or Serperior-both of which Seviper beats handily, so that's something. It probably won't beat anything else on his team after that, but it can at least Glare and get a decent hit off on whatever he sends out next.
-Shantaul: Despite her team being fairly slow and weak to Crunch, Seviper still struggles a lot here, not being able to beat anything except maybe Jellicent 1 on 1.
-Grimsley: You might be able to set up Coils on Liepard, but this is hard since it has a high crit-rate with Night Slash, and Attract is a complete pain in the ass if applicable. You'll need Aqua Tail (or Rock Smash if you don't want to spend shards on Seviper) to deal with Krookodile and Bisharp. This opens up moveslot issues, as this would make the set Poison Jab/Crunch/Aqua Tail/Coil, which leaves no room for Glare, which is the only thing Seviper is going to do versus Iris. Honestly, I think the best option would be to forgo Crunch for Glare to optimize Seviper's performance for the Elite 4, as Glaring Iris' Hydreigon is much more useful than Seviper's miniscule contributions versus those Shantaul and Caitlin. Granted, this means you would need to forget Giga Drain if you had learned it (which is annoying for a move you can't relearn easily), but objectively speaking for the purposes of tiering, the game is over once you beat the Elite 4, and Giga Drain doesn't help much there.
-Caitlin: Seviper is in a pretty similar situation here as it is with Shantaul, except worse since obviously anything on Caitlin's team can OHKO it.
-Marshal: Seviper sets up Coil easily on Throh, and can then proceed to sweep Marshal.
-Iris: If you have Glare, you can get off to a decent start by using it on her Hydreigon. Otherwise, Seviper loses miserably to her entire team.
Additional Comments: Seviper has fluctuating EXP growth, which has by far the most amount of level 100 exp, though thankfully I don't think it really takes off until the later levels. Still, it's yet another disadvantage Seviper has to deal with.