Fishing: I'm just fishing to fill my bag up and then releasing them. I don't spend as much time on my game anymore but when I'm doing daily chores if I see a fish I'll catch it. Fish in the ocean, they seems to spawn better there than the river. Im catching at least 16 everyday until I get gold. I'm on silver because I used to go to the island to catch rare fish and sell them for bells, but I don't need bells anymore so I'm just trying to keep up the number of fish purely for completion now.
Bug Catching: I already have gold but I caught bees everyday. There should be 5 in your town, but it could be less depending on your number of trees and stuff like that. I also used to beetle hunt at night so that was helpful. Catch every single bug and release the cheap ones or sell them when your bag is full and repeat until your basket is filled. Thats probably the best way. Island tours do not count towards this, fishing, or diving. catching and releasing does count, you don't have to swap for it to count.
Diving: UGH, just keep at it. I aim to catch at least one scallop for Pascal a day so sometimes that takes a long time because those buggers are so fast. I usually take about an hour and do maybe about 3/4 bag full. Good thing it's only 1000 though instead of 5000 like the other two.
Encyclopedia: just keep a list of everything. Check the seasons and times. Ask people on the train station board to TT to certain times you might need or have missed.
Balloon popping: pop every balloon you see. The silver shovels will count as two pops. Just keep an eye and ear out for floating balloons.
Medalist: go on tours with friends. If not that, go on tours with people from ct. Do tuna kahuna/giant dragonfly. It's the best one in my opinion. I did the tuna tour for one whole night with a group of people and got the silver the next day after I just finished with getting the bronze two days before.
Gardening: plants lots of fruit trees. Plant bushes, trees, and flowers from bags. As long as they have been planted, can can dig them back up or destroy them etc. i got it when I officlaly started landscaping and planted my cherry orchards and Phineas popped up when i started back up after I had gotten back from school.
Weeds: ask time travelers, usually they need their towns cleaned the most. If someone has a cycle town, they might need a clean up crew.
Bellionaire: save the money from all that bug hunting. Sell stuff at retail on here and in game lol. Win giveaways. Sell turnips. Sell TBT.
Stock market: make a second character, use them to buy turnips, and sell using the character you need the badge for. This is all about profit so if you buy 1millions worth and sell 3millions worth you net profit is only 2 million.
Shopper: buy everything you like. I'm pretty sure buying from other towns count too. I brought the crown and royal crown in people's towns before since I needed it for catalog and it never appeared in my shops, next day I got the badges. This can be worked up to though especially if you are wanting to complete your catalog.
Refurbish: refurbish the cardboard box. It's cheap and refurbishing back to normal also counts as 1. I did this by set. I wanted a gold cabana set so I refurbished that and I wanted a silver modern set so I refurbished that. Anything that would look nice refurbished I would do the whole set and place it in my house. It was pretty nice looking and I got to reach a high HHA score because of that. Two birds with one stone yo lol Also it's not a full 30 minutes, I think it was around 24 minutes so yeah putting that info out there. Less time spend is better I say.
Catalog: just know that it's more than 80% to get the gold badge. I used
http://animalcrossingnl.mooo.com to keep everything in check. I used
http://moridb.com to make a wishlist of all the stuff I still needed and posted a thread on the retail board to get everything.
Happy Home: this one is easy. However, if you want the gold exterior stuff, don't do the themes. (My opinion obviously, I never got higher than 100k and I matched the theme and everything so idk) I just placed sets in all of my house and placed villager pictures in my rooms. The themes didn't really help me. If you find that you are using the theme challenge and your score is stuck, just don't do the theme stuff. That's basically what happened to me.
KK: just keep asking him to play songs. If you want his picture, fill your bag with things so that he can't give you a song. Keep doing that 20 times and the next day his picture will be in the mail.
Requests: just keep doing things for your villagers. This is the easiest one. Keep talking to them and they'll always have something for you to do. Plus you'll be able to check who's moving that day and build friendship c:
Letters: I wrote 10 villager letters once everyday. I mainly just put "Hi friend!" On four different lines and got the badge when I was all done. You can buy letters beforehand and calculate how many you need left. It's pretty simple.
Streetpass: good luck!... Go to best buy, Starbucks, McDonald's, Home Depot, lowes, the mall, university, everywhere. If you live in a place that has a lot of people who own a 3ds thats even better. I don't have that luxury and it took me a year to even get bronze so I'm not sure how this gold badge thing will work.
Traveler/host: sell things, do giveaways, ask for help in train station board. You can check how many people you need left for host by sitting on the tree. You can check how many people left you need to visit by asking someone to visit your Dream town and ask porter how many times you've traveled (or visited if your tree isn't big enough to sit on)
Dreamer: the fastest way. Grab however many bells you need for 500 dreams, cost is 500 bells each. Watch a movie, grab some popcorn, and just let your hands do the manual work by pressing A and pushing the circle pad left. If you want I suppose you could always visit really nice towns too to get inspiration and stuff, but if it's for the purpose of the badge purely the first way is the fastest, it's tedious though in either case.
Town rep: i heard that you can leave your game on while you are in the ocean, you'll still be counted as playing. Idk about that, but if you played as much as me, I'm pretty sure this badge will be a breeze
This blog
http://acbadgehunter.tumblr.com/tagged/acnl-badges also has some tips I might have missed. They don't have every single badge listed with guides but some stuff are pretty good to know.