I think the choices for partners on here have been the least helpful throughout the 5th gen Pokemon of the Week. For partners everyone list types of Pokemon that can deal with the POTW's weakness, but that's not how you pick a partner in double/triples battles, because you're going to be taking Earthquakes, Surfs, Heat Waves/Lava Plumes, Discharges, Blizzards and more.
A true example of a partner would be Ninetales, Tangrowth/Exeggutor, Terrakion/Aerodactyl. This team completely supports one another. Ninetales Drought ability boost it's fire type moves, making Heat Wave a huge threat, as well as, getting rid of the teams water weakness.
At the same time it's drought ability doubles Tangrowth's/Exeggutor's speed thanks to Chlorophyll making them great sweepers. This also cover Ninetales water, ground, and rock weaknesses, with Ninetales covering it's Bug and Ice weaknesses. Tangrowth is the better choice IMO for being a mixed sweeper.
Finally Aerodactyl rounds things out by taking care of Tangrowths other weaknesses, leaving only an Electric weakness uncovered.
This is how partners work, by completely benefiting/covering one another, not just being able to take on a type the POTW is weak against.
When looking for a partner you have to look through ALL 649 Pokemon, and find the perfect chemistry between them.