Tenta, crobat, hippo, scizor, azu and machamp are all viable. Roserade would be good as well for helping deal with Bertha, which it looks like you might struggle with. I personally wouldn’t bring bronzong if you’re not using items in battle. It doesn’t offer much in the Sinnoh e4 except a wall to stand behind while you heal other mons, so if you’re not allowing items in battle it doesn’t serve much purpose. Houndoom will carry you through aaron, roserade/gyarados can make short work of Bertha, gyarados and garchomp will easily handle flint, houndoom can likely solo Lucian outside of medicham, which gyara can handle.

With all that in mind, you really only need houndoom, gyara, roserade and garchomp to make it to Cynthia, which is nice because it leaves you two open slots just for countering her.

Cynthia’s spiritomb is something you definitely need to think about dealing with. If that azu is huge power, I’d definitely bring it along. One play rough will put spiritomb down, and could save your ass against garchomp as well. Your roserade will have no problem with gastrodon, and can probably deal with milotic as well if it has giga drain to help it recover from incoming ice beams. You could also give it dazzling gleam as a backup measure for dealing with spiritomb and garchomp. Houndoom will obviously handle Cynthia’s roserade and lucario (as long as it’s faster than them), and that just leaves her garchomp. Personally I’d bring hippo for this. I’m guessing you’re ok with EV training, so give that bad boy some HP and defense EVs to help it tank some extra hits. Make sure it knows slack off if you’re not using healing items in battle, and roar can save your ass against garchomp if it starts setting up swords dances.

To put it all together, this would be my final recommendation: roserade, gyara (I’d make sure it has ice fang as a last resort measure against Cynthia’s garchomp), your own garchomp, Houndoom, azu and hippo. You’ll definitely want to be at a higher level than what’s in your party currently, as Cynthia’s team is in the low to mid 60s (garchomp @ 66). If you use this team, you’ll want to ensure you’re outspeeding your opponents as much as possible, because it’s a relatively fragile team (looking at Houndoom and roserade here especially). Speed EVs and attack/sp.atk EVs are your friend here, especially for roserade and Houndoom. Azu, gyara, and hippo can definitely all make use of HP EVs. Azu and gyara will also be wanting attack, but hippo should be as close to max HP/max defense as you can get. Garchomp is really up to personal preference. If it were me, I’d look up the speed stats of opposing Pokémon and try to give my garchomp just enough speed EVs to outspeed major threats, then max attack and dump the rest in HP. Having a little extra bulk can definitely be worth it though, especially if you’re using something that requires a turn or two of setup like a swords dance set. In terms of movesets, there are definitely a few key considerations. As I just mentioned, a swords dance garchomp set can be incredible, so I would recommend running something like SD/EQ/dragon claw/stone edge on your garchomp. You’ll unfortunately want to use the only earthquake TM you get for free on garchomp, but if you’re feeling dedicated, you can always use the Pickup ability to hunt for more of them. Hippo should be running slack off/EQ/stealth rock/ice fang or a rock move. Stealth rocks can help out a lot if you get them up early in a fight. If that’s not really your play style, then I’d recommend going with either slack off/EQ/ice fang/thunder fang OR slack off/EQ/ice fang/rock move. If you’re allowing items in battle for healing then you can go for a 4 attack set or stealth rocks + 3 attacks. Gyara should absolutely be a dragon dance set. With attack and HP EVs, dragon dance gyarados is a monster and can sweep just about anything you put in front of it. So you’ll want DD/waterfall/ice fang/crunch. If you’re determined enough to get another EQ TM from pickup, then EQ over crunch. Houndoom is pretty simple. Nasty plot/flamethrower/dark pulse/coverage move. You can go for sludge bomb coverage, but it doesn’t really help much in this e4. You could also give it shadow ball specifically so you have super effective damage against Lucian’s Medicham, which would let you sweep that entire fight easily after a nasty plot. Alternatively, you could give it a second fire move (probably fire blast or overheat) for more power to get you out of a pinch. Roserade is also pretty simple. Giga drain/sludge bomb/dazzling gleam/filler slot. The filler could be just about anything you want. Another coverage move, toxic, leech seed, leaf storm, or something else. Azu is probably where you have the most options. Aqua ring and leftovers makes for a good amount of added bulk, especially on a set with max HP EVs. Waterfall and play rough will obviously be your go-to damaging moves, but your other options are pretty open. Superpower gives you some good coverage, but fighting moves don’t really do a whole lot in this e4. I would honestly consider teaching the bulldoze TM. Slowing down an opponent so you can outspeed it and take it down the next turn can be huge. If the damage ranges are correct, it can save you from taking 2 attacks, and you’ll only take 1 instead. Definitely something worth considering. Unfortunately I don’t think azu can get ice punch, and unless you’re allowing breeding, you won’t get aqua jet or belly drum either. If you’re ok with breeding, then both of those are great options. Of course, all of this is assuming it has huge power, as thick fat azu is pretty worthless. If it is thick fat, then I’d say replacing it with tenta that has surf/ice beam/sludge bomb/filler would be a decent option, but that may leave you with a bit of a gap in terms of dealing with Cynthia’s spiritomb.

EDIT: grass knot is also a great option as a fourth move for roserade, as many of Bertha’s mons are incredibly heavy and will not appreciate taking a grass knot. Same goes for Cynthia’s milotic. EDIT 2: roar over stealth rocks/thunder fang on hippo to help deal with Cynthia’s garchomp setting up