I can help you with the Gym Leaders part. The first Gym (Cilan/Chili/Cress) is always chosen to have the advantage against your starter, so for example choosing Tepig makes you face the Water-type user Cress. You can pick up the elemental monkey from an NPC in the Dreamyard who can beat that Gym's own monkey - you get Panpour if you chose Snivy, Pansage if you chose Tepig or Pansear if you chose Oshawott.
Second Gym (Lenora) is notably difficult due to her hard-hitting Normal-types and their Retaliate strategy. The Watchog also likes using Hypnosis, but thankfully Cheren gives you Chesto Berries when you get to Nacrene City.
Third Gym (Burgh) isn't very difficult, since Bug-types have a lot of weaknesses. His Dwebble is often the hard one to take down, since it doesn't have any of the common weaknesses for Bug, but Water can still beat it. Fire and Flying moves easily take care of his powerful Leavanny.
The fourth Gym (Elesa) is one of the more infamously hard ones of B/W, since she has that annoying Volt Switch strategy with two Emolga, and also has that hard-hitting Zebstrika. If you have a Ground-type like Sandile and are packing the Rock Tomb TM (found in Desert Resort) you should be set.
Fifth Gym (Clay) is also notably hard, but that's because of his ace Pokemon Excadrill being a very strong physical attacker in itself. His other two Pokemon aren't as difficult thankfully.
Sixth Gym (Skyla) is one who can easily be destroyed by Electric-types, since all her team have a common weakness to it. Something like a Zebstrika (which you can catch just nearby on Route 7) can easily mow her down. Her Swanna can hit very hard if you don't have such advantage though.
Seventh Gym (Brycen) is the easiest of the game, since you're very likely to have something that can easily destroy Ice-types, and with four common weaknesses (Fire, Rock, Steel, Fighting) it's not surprising. If you picked Tepig, your Emboar is sure going to have a field day with this.
Final Gym (Drayden in Black or Iris in White) uses Dragon-types, and tend to be spamming Dragon Dance to set up potential sweeping against you, but they also like using Dragon Tail to make you switch randomly. If you have an Ice-type like Beartic or the Vanillite line, you can pick at that Ice weakness, but otherwise use your strongest non-resisted attacks.