I think it might be easier for me to just list sets that I think might work better (this is assuming it's gen 6):
Bisharp @ BlackGlasses
Ability: Defiant
EVs: 248 HP / 252 Atk / 8 Def
Adamant Nature
- Sucker Punch
- Knock Off
- Iron Head
- Swords Dance
Bisharp works best as a sweeper. It has huge attack and great offensive moves, so a Swords Dance set is usually the most effective on him. Set up a Swords Dance, Sucker Punch anything faster (or predict they use a status move then use Knock Off or Iron Head instead). Iron Head is there for an extra STAB and more coverage, especially against fairies and rocks. BlackGlasses are there to add a bit more power to Sucker Punch and Knock Off, which can come in handy sometimes.
Talonflame @ Choice Band
Ability: Gale Wings
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Brave Bird
- Flare Blitz
- U-Turn
- Me First
Choice Band Talonflame is truly a beautiful thing. Gale Wings allows you to spam priority Brave Bird, a STAB Flare Blitz is great, U-Turn adds momentum and Me First... does whatever Me First does. You won't be using Me First much, but if you think it could come in handy you might as well try it. If you want you can put in something like Aerial Ace instead of Me First for a flying move that has no recoil, but it isn't very strong- it is stronger than your standard Aqua Jet or Quick Attack, though.
Venusaur @ Venusaurite
Ability: Overgrow
EVs: 248 HP / 8 Atk / 252 SpA
Modest Nature
- Earthquake
- Giga Drain
- Sludge Bomb
- Synthesis or Hidden Power [Ice]
Mega Venusaur is made to tank hits. It's also made to deal damage. Primary STAB is Sludge Bomb, Giga Drain is a secondary STAB that does a lot of damage and also heals. Synthesis is there for a more rapid recovery that will take effect immediately, but Hidden Power [Ice] hits dragons for supereffective damage, like Dragonite or Garchomp. Earthquake hits the types that wall Mega Venusaur such as fire and steel.
Tyranitar @ Chople Berry
Ability: Sand Stream
Shiny: Yes
EVs: 248 HP / 252 Atk / 8 SpD
Adamant Nature
- Protect or Stealth Rock
- Low Kick or Superpower
- Crunch
- Rock Slide or Stone Edge
Chople Berry on Tyranitar can be a huge help. It won't always survive the attack, but sometimes it will leave you with a decent enough chunk of health. Sand Stream boosts it's Special Defense and deals some risidual damage, which is nice. Protect's job is to scout for attacks, and help stall an extra turn or two so the sand runs out and your next Pokemon isn't taking damage when T-Tar dies, while Stealth Rock sets up entry hazards. Low Kick and Superpower are there for fighting coverage, mainly to hit Heatran and opposing Tyranitar. Low Kick has no negative effect on the user, but does less damage to lighter opponents, while Superpower always hits for solid damage but lowers your attack and defense by one stage. Crunch is your primary STAB, and Rock Slide/Stone Edge is your secondary STAB- pretty self-explanitory. Which one you choose to use in the last slot is the omnipresent power vs accuracy.
Rotom-Wash @ Choice Scarf
Ability: Levitate
EVs: 252 Spe / 252 SpAtk / 4 HP
Timid Nature
- Hydro Pump
- Volt Switch
- Trick
- Thunderbolt
Choice Scarf Rotom-W is great for hit-and-run, or just doing damage. Hydro Pump is a great STAB move that dishes out chunks of damage, and Thunderbolt is great if you'd rather KO reliably than just hit with Volt Switch and go to the next Pokemon. Trick can cause trouble for walls and pokemon that are setting up absolutely anything, like Swords Dance or entry hazards.
Now, I'm not going to suggest a set for Manectric; you already have Rotom to fulfill it's role, and Venusaur is your Mega. Instead I'll suggest a Pokemon that fits better into your team so you're not left with type redundance.
Latios @ Life Orb
Ability: Levitate
EVs: 252 Spe / 252 SpAtk / 4 SpD
Timid or Hasty Nature
- Draco Meteor
- Psyshock
- Hidden Power [Fire] / Surf / Earthquake
- Defog / Roost
Life Orb does what it always does; boost damage. Keep in mind that it does around 10% recoil whenever you attack, however.
Draco Meteor does damage, and a whole lot of it. STAB + Life Orb + 130 base power = death. However, it drops your special attack by two stages, so use it wisely- you'll almost always have to switch after using it. Psyshock is a good STAB move that uses your special attack and your opponents physical defense, meaning it does a lot more damage to a special tank or wall like Blissey than Psychic would, despite the 10 base power drop.
Hidden Power [Fire] hits steels, bugs, and grass types, meaning it's great to fend off Ferrothorn, Jirachi, Skarmory, Scizor, Celebi, etc. for supereffective damage. Surf hits fire types like Heatran, ground types (most notably Gliscor), and rock types (even most Tyranitar will be 2 or 3HKO'd). It also OHKOs Excadrill and Landorus. Earthquake does a lot of damage to Heatran (usually an OHKO), and does good damage to other steels and fires.
Defog is best used if you chose Protect over Stealth Rock on Tyranitar, while Roost does better if you haven't set rocks up yet- Defog gets rid of screens and entry hazards (and lowers the opponent's evasiveness, so you're less likely to miss with Draco Meteor if you use it after), but it also gets rid of the ones you've set up. Roost is just there to heal the risidual life orb damage and damage you may have taken from the opponent.
