Sceptile can be a killer if used right. If used wrongly, it's low defences will prove to be his destiny against things like Heracross' megahorn, Metagross' Meteor Mash, Gyarados' return etc.
Sceptile @ Leftovers
Timid nature
Trait: Overgrow
-Leaf Blade
-Thunderpunch (available from move tutor in Emerald)
-Leech Seed (breed with any grass type that knows this except Celebi)
-Substitute (There is a tutor for this in emerald)
Subseeding Sceptile, effective, especially with Sceptiles amazing speed. The sub hp loss is easily recovered with leech seed and leftovers, and if your opponent doesn't manage to destroy your sub in one turn because of a switch, or missing an attack, attack yourself.
You can also go with
Sceptile @ Petaya berry/Leftovers
Timid nature
Trait: Overgrow
-Leaf Blade
-Thunderpunch
-Crunch (Breed with Tyranitar, Salamence or Seviper)
-Dragon Claw (TM or breed from Salamence)
Sceptile loses a fair bit of power in 4th generation though, with it's main attacks all becoming physical. Crunch, thunderpunch, leaf blade, and dragon claw all become physical, which does sceptile no favours.
For fourth generation, Sceptile loses alot of it's special movepool and becomes a little bit left off unfortunately. With it's only special attacks remaining being Grass Knot, Leaf Storm, Frenzy Plant, Energy Ball, Solarbeam, Giga Drain, Mega Drain, Absorb, Focus Blast, Hyper Beam Dragonbreath, and Dragon Pulse it can't use it's special attack well. Note that all those moves are only grass, dragon moves, one inaccurate fighting move, and one worthless move that needs a recharge turn, and dragon doesn't really hit alot of super effective damage.
You can go for a special sweeper moveset, but it isn't very effective.
Sceptile @ Choice Specs
Timid nature
-Leaf Storm/Energy Ball/Grass Knot
-Dragon Pulse
-Focus Blast
-Hidden Power Ice
Note that Dragon Pulse hits only Kingdra for super effective damage while Hidden Power doesn't.
It can go for some kind of physical moveset, but it's physical power is alot lower than it's special power. It still has some hitting power however with Adamant and a Choice Band
Sceptile @ Choice Band
Adamant nature
-Leaf Blade
-Thunderpunch
-Earthquake
-Rock Slide
-Crunch/X-scissor
The final move is basically filler. Both have the same base power, both hit psychic types for SE damage. One is damaging to dark types, but resisted by ghosts, the other is damaging to ghosts but resisted by darks. X-Scissor hits grasses for SE damage but is resisted by fliers, which Crunch hits both for normal damage. Both are also resisted by fighting types.
You can also go with a swords dance sweeper, if you manage to set up
Sceptile @ Leftovers
Jolly nature
-Swords Dance
-Leaf Blade
-Earthquake
-Rock Slide
It has sweeping potential. After 2 swords dances it will get in the OHKO range against alot of opponents.
As opposed to Sceptile, Absol gains alot in the 4th generation. In the third generation it still belongs in the range between NU and UU though without physical STAB, no defences, a slow speed and an ability that is useless without any defensive power. It's only good thing is baton passing
Absol @ Salac Berry
Jolly nature
-Return
-Swords Dance
-Shadow Ball
-Baton Pass
set up swords dance in the turn your opponent attacks and brings your hp way down, activating salac. Use salac boost to pass out before next attack.
In 4th generation Absol gets a good boost, with an expansion of its physical movepool, physical STAB, and a better ability
Absol @ Scope Lens
Trait: Super Luck
Jolly nature
-Night Slash
-Psycho Cut
-Stone Edge
-Double Edge
DE is there for raw power, the rest is for type coverage, and has high critical hit ratios. With Super Luck and Scope lens increasing those odds even further you will have alot of critical hits. There are still alot of pokes walling these attacks, like most steel types, most notably Steelix. It still has low speed and defences, so it isn't going to last long. It is a letdown it doesn't learn ground attacks, it could've used them well. It also doesn't learn fighting moves to deal with steels, except for Rock Smash.
For Manectic, things have changed alot to the fourth generation. It gained the ability to use Fire moves for some reason, but lost Crunch as a special move.
For third Generation, this is a potential set
Manectric @ Petaya berry
Timid nature
Trait: Static
-Thunderbolt
-Crunch
-Thunder Wave
-Hidden Power Ice/Substitute
Sweep and wave fast pokes that you want others to deal with, like pokemon with a good special defence that rely on speed
For fourth generation, this can be decent
Manectric @ Leftovers/Petaya Berry
Timid nature
Trait: Static
-Thunderbolt
-Flamethrower
-Hidden power Ice/Substitute
-Thunder Wave
The addition of Flamethrower is a big bonus if you can't get Hidden Power Ice, since it allows you to hit grass types for SE damage
Blaziken got a boost in the generation shift, gaining physical fire moves.
For third generation, go with the boring
Blaziken @ Leftovers
Jolly nature
-Overheat
-Rock Slide
-Sky Uppercut
-Swords Dance
For fourth generation, go with
Blaziken @ Leftovers
Jolly nature
-Blaze Kick/Flare Blitz
-Stone Edge
-Sky Uppercut
-Swords Dance
Blaze kick for no recoil, flare blitz for power.
Banding Blaze
Blaziken @ Choice Band
Jolly nature
-Blaze Kick/Flare Blitz
-Stone Edge
-Sky Uppercut
-Thunderpunch
Thunderpunch is a nice addition for Blaziken, hitting water types and flying types super effective as long as they aren't called Swampert or Gliscor
Blaziken got alot of new toys in generation shift. Night Slash, Shadow Claw, Stone Edge, Brave Bird, Poison Jab, Will-O-Wisp, Agility, Baton Pass, physical Blaze Kick, thunderpunch and Flare Blitz. Brave Bird and poison jab aren't useful though, as they don't hit anything notable that fire doesn't hit, except maybe ludicolo. Shadow claw and night slash are basically covering the same, with the one being resisted by Fighters and the other being completely walled by normals. Will-O-Wisp, Agility, and Baton Pass are hard to fit on the sets
Fourth Generation sets in Italics