Firstly, this looks really good for a first competitive team. Also, very good post overall, with all ev's and stuff and stating what kind of help you want. I wish everyone did it like this
You already saw your fighting weakness, and this is truly an important flaw currently. At this moment, the metagame is filled with fighting types and the most common priority move (mach punch) is also fighting-type. Atm, a Breloom can sweep all these three mon with technician mach punch and bullet seed. Therefore, you need something that can take on Breloom.
Celebi is the ultimate counter to Breloom, resisting both STABs and not being crippled by Spore due to Natural Cure. It outspeeds Breloom and can OHKO it with Psychic or Hidden Power Ice. The most common Celebi set is a specially defensive one, but you can also go for an offensive one. When you go for the specially defensive one, make sure to put enough speed ev's to outspeed Breloom though. Celebi is also a fantastic counter to rain teams, especially the likes of Keldeo and Rotom-W. It has good defensive synergy with your Swampert too.
There are not many threats that can sweep your whole team, mostly because of Terrakion, which can eliminate anything that is trying to start a sweep and hasn't got many speed boosts yet. Scizor, the most used mon in OU atm, can easily beat your Terrakion with bullet punch, but is handled by Swampert and I think Hydreigon can take a bullet punch when at nearly full health and OHKO with Fire Blast.
Once your Terrakion is gone, your team can be swept fairly easily though, because there are a lot of pokes in OU who can outspeed Swampert, Hydreigon (especially since it hasn't got a speed-boosting nature) and Celebi. I suggest putting in one more really fast mon, preferably with 110 base speed or more.
I personally like focus sash Alakazam, as it is guaranteed to get one attack of when brought in safely. Dugtrio is a good option too, eliminating many Pokemon you have problems with. Those two are very frail though. Other options for fast pokes are Latios, Latias, Espeon, Jolteon and Starmie. There are probably some more that I can't think of, though.
For the last Pokemon, it can be anything, really. Just think of something your team needs, such as a rapid spinner, priority user or a cleric, and try out some things, I suggest. Maybe something to clean up late-game. Dragonite is a good example, being able to sweep a whole team after just one dragon dance. It has multiscale too, which is a very good ability.
I hope this helped, and if you have questions, feel free to ask them!