Pokemon City

http://mycheats.1up.com/view/section/3172942/28734/pokemon_heart_gold_soul_silver/nintendo_ds
I think knowing what Pokemon they will send out first might help you (saves you switching out or wasting a turn) for a start.
I have some advice for you that might help, but it's upto you to follow it or not of course. xD

For Meganium, I think it would be a good idea to get rid of Flash. (Use HM slaves for those if you need it for caves)
Light Screen is an awesome move, so maybe investing in getting Reflect is a good idea to protect against the massive damage those Outrages (and other attacks obviously) do.
I think it is worthwile also to get rid of Razor Leaf and replace it with Seed Bomb (once you get the chance).
It can be the tank of your team, come in and set up screens and switch out and let your attackers do the damage.

Imo your Gyarados is perfect!

Dragonair's set is looking good, but you have 2 dragon moves. It is generally a good idea to get as much coverage as possible in case you get faced with a type you're not expecting.
Since Dragonair's greatest stat is its Attack, you will want to invest (even if not now) into learning Outrage.

PS. If I recall you get a Dratini at that shrine (Blackthorn city I think?) where the dragon type leader is. That Dratini knows extremespeed! Maybe later on you can breed those 2 and get an uber Dragonair with Extreme Speed, Outrage, Dragon Dance and Aqua Tail?

For Infernape it might be an idea to get rid of Flame Wheel for Flamethrower (TM) or Overheat (also a TM). Infernape has great Special Attack and Attack so you can utilize both which is great.
I highly recommend teaching it something like Grass Knot or Thunder Punch in case you are in a pinch with a water type!

Tyrannitar is in a similar boat as Dragonair. It has 2 attacks of the same type, you can free up one of those slots for more coverage. Since his SpA is alot lower I'd get rid of Pulse. Replace it with something like Fire Punch or Earthquake?
Then the last slot you can decide to keep Thrash or replace that with the likes of Dragon Dance, Aqua Tail, Ice Beam, Flamethrower etc. Upto you!

For Noctowl, invest in Roost, it is so useful! His Attack stat is detrimental so you might want to replace things like Air Slash with Night Shade. Since you don't have status effect attacks on anyone on your team, you might want to learn it Toxic. It can be killer. (A mean strategy is Toxic and Whirlwind. Use it on the first Poke, then Whirlwind forcing him to switch, then toxic that one until his entire team is badly poisoned XD).

Hope I was helpful and SORRY for this being like UBER LONG.

Zuruggu (forgot the English name, it's the Dark/Fighting one)
Who likes to Leer then Headbutt. one of my fav pokemon in that anime, next to Iris' Emolga.