Just one Fighting-type move will do. I also find Earthquake useful as it can deal heavy damage to most Electric-types or opponents like Skuntank or Drapion, and isn't a direct attack (no worries about Pokemon w/ Static). That, and Kangaskhan is the only member of my team that can use it. It's risky in a double battle -- but I'll likely use it if my Cloyster uses Protect (not consecutively, of course) or if my Crobat is on the field (just need to be concerned about opponents with Gravity).
Thanks for your suggestions, but I think I'll go with my third option.
My Flareon has Return in her arsenal, so I have my favorite Pokemon to take out certain opponents with "tough love". As for support moves... Gardevoir has Safeguard to keep my team safe from status conditions (such as Poison, Sleep etc.) for five turns. Banette has Embargo, which works well with his Frisk ability. Crobat has Confuse Ray, and Cloyster has Toxic Spikes.
Brick Break isn't as powerful as Focus Punch, but it does come in handy for opponents who annoyingly use Reflect or Light Screen. I'd been thinking about that for a long time. Plus it's 100% accurate, while Hammer Arm has 90% accuracy (Rock Slide also has 90% accuracy, so I don't think it's a good idea for a Pokemon to have more than one attack that's not always accurate).
I don't rely on the most powerful attacks; but moves like Flamethrower (Flareon), Water Pulse (Cloyster), or Psychic (Gardevoir) can get the job done. Helping Hand can give these moves an edge in a double battle. I know because I recently teamed up with Marley on Victory Road, and my attacks were more powerful each time her Arcanine used it. In single battles, Kangaskhan would still have three different move types to work with.
Going to try and breed/catch a Kangaskhan with a Brave or Adamant nature. I just need to get lucky.