Blaziken is very good. If you SR to get one with speed boost and good ivs, it can potentially sweep many gyms.
Pixiliate Sylveon with Hyper Voice and Misty Terrain is given to you after beating grass gym.
Aevian Ampharos is found after you show Zumi a Mareep in Chapter 8
Aevian Mismagius is found in Darclight Woods, you need leaf stone
Sheer Force Nidoqueen can set up toxic spikes and cleave through teams (Use moon stone at level 35- learns t.spikes as nidorina)
The last slot should be rotation depending on the gym.