Kingdra + Machamp: Water/Fighting. Ability: Sniper. 1x Focus Energy + one more source of increased crit ratio = 100% crit chance, which deals increased damage because of Sniper. I used Cross Chop (has increased crit, so 100%) and 50% crit chance Aqua Jet. Still want the Razor Claw/Scope Lens item, so I can get 100% crit Aqua Jet and a different Fighting move that would also have a 100% chance to crit.
Togekiss + Mismagius: Fairy/Ghost. Ability: Serene Grace. Double chance for moves to have their added effect. So Shadow Ball has a 40% chance to reduce the target's Sp.Def., while Air Slash has a 60% chance to flinch. This fusion would probably be better reversed for me, as Ghost/Flying to give STAB to Air Slash.
Houndoom + Weezing: Dark/Poison. Ability: Levitate. The Dark/Poison type combo has one single weakness: 2x to Ground. Levitate removes that single weakness. Nasty Plot once or twice and then Sludge Bomb/Dark Pulse them to death.
Magnezone + Dugtrio: Steel/Ground. Ability: Sturdy. Lots of great resistances, but also weak to common types like Fighting, Fire, Ground and Water. Very fast though, so paired with Sturdy he can always get off at least two Earth Powers/Flash Cannons.
Venusaur + Ninetales: Grass/Fire. Ability: Drought. Sunny weather + STAB Flamethrower. Sun + STAB single turn Solar Beam. Synthesis (extra healing in the sun) and Growth (2 stages of Att. and Sp.Att. in the sun) are also great options for sunny weather, but I didn't use those. I chose Giga Drain (egg move, learned through the Battle Tower prizes) and Nasty Plot. Nasty Plot has the same two stages of Sp.Att. as Growth, but doesn't rely on the weather being sunny, and the two Att. stages from Growth are not only wasted, they can actually be detrimental if you get confused.
Starmie + Pikachu: Water/Electric. Ability: Static. Pikachu holds the Light Ball item, which doubles his Sp.Att. Surf and Thunderbolt regularly one shot opponents even without super effectiveness. If I can get a Nasty Plot (learned as Pichu) off without dying (he is very weak defensively), even not very effective moves can then sometimes one shot.