Curse, Leech Seed, Synthesis, Light Screen, or some other support/defensive/healing last move to help its durability, especially against physical attackers. Could have Curse + Synthesis, but that would mean sacrificing Rock Slide, and Rock is fantastic coverage on Torterra so probably not a good idea.
Learns Brave Bird at level 49, so you can get rid of Fly. It currently has two Flying type moves, both kinda meh for the late game. And it's missing Normal Stab, Return is an easy pick there since with max friendship you have a reliable 102 base power move. Doubt Roost would help much since Staraptor's defenses are awful and can very easily lose more than half HP in one attack. U-Turn is a great move for it since it allows an extra hit while switching to something that resist an upcoming attack.
Physical attacker but lacks strong physical Electric move. Thunderbolt + Thunder Fang is kind of weird but sadly there isn't much you can do about that in Gen 4, Luxray's move pool is just poor.
Embargo seems pretty pointless since the AI very rarely use items. Nasty Plot makes Houndoom a fantastic in-game sweeper, but I guess that's not an option because it won't learn it until much later. Sunny Day + Solar Beam could work well, also boosting its Flamethrower in the process. Thunder Fang isn't great coverage, it's a weak move and Houndoom isn't even a strong physical attacker, so most of the time a Stab Dark Pulse would do more damage against Water types than a Thunder Fang, and you could also have Solar Beam to help with that.
Crunch seems a bit redundant since if you want it to take care of Ghost and Psychic types, you already have Stab Shadow Ball for that.
Thunderbolt TM would be great coverage, but I guess you used yours on Luxray already unless you have more than one. Otherwise, maybe Thunder Wave, Light Screen, or something else for support. Rain Dance + Thunder is also an interesting combo but maybe too situational.
Clearly a special attacker so I asume Waterfall is there just because it's an HM, should get replaced with something more useful once you no longer need it.
Ice Beam is obviously great coverage, but you probably already used yours on Froslass and Blizzard is not so reliable. Not much else you can do for it on the attacking side, but since Milotic is a great Special tank, moves like Recover and Aqua Ring may help. Seems to have nice synergy with Torterra.
Can't say much about held items because I don't recall which ones are available before the post-game. Generally Stab boosting items (like Charcoal / Black Glasses Houndoom or their equivalent Arceus Plates depending on the opponent) works fine. You can also have Lum Berries, or Sitrus Berry on the more defensive Pokémon.