With halo.bungie.net going down Feb 9th, there's been a scramble to save data. While community efforts are underway on that, I thought I'd share the best way I've found to save a copy of the Halo data for your Gamertag(s). I tried to make this idiot-proof, so keep that in mind as I over-explain things.

IMPORTANT TIPS:
-You likely won't be able to fit all your stats into one run of the program. That's fine, you can run it many times and combine the files later.
-I suggest you run the program multiple times to account for bungie.net errors. Between the different copies you should get all your data. You can combine the files later, but for now just make sure you have everything.
-This process can take a while. Don't wait! The deadline is fast approaching. I give many tips to speed things up.
-Don't run the program more than two times at once. That tends to make bungie.net give more errors. Slow and steady.
-Backup your files. I describe my preferred way at the end.

First download the .txt files I provide here. They'll make the process of entering the URLs much faster and easier. Use Notepad to read them.
Next install HTTrack.
Once that's installed, open it up, click "Next" and you'll see this screen: Name the project whatever you want. The format I use is "B.net GAMERTAG." Category doesn't matter. Put anything, or leave it blank. I put "b.net stats". After that, click "Next".

On this screen, we set the pages to download (1). There's a generous character limit, but if you're like me, you played more than enough to surpass it. If so, you have to split your stats into multiple projects (I suggest every game have its own file). We will set it to download every page of your game history, which links to every game you played. First thing to do is set the limits and exceptions. It isn't strictly necessary, but it significantly speeds up the download. Click the "Set options..." button (2).

An options box will pop up. Click the "Limits" tab, and you'll see this. Set the "Maximum mirroring depth" (1) and "Maximum external depth" (2) to "2". You can do more, but that will dramatically increase the size of your download, and how long it will take. Now click "Max transfer rate (B/s)" (3). Set it to blank, which will make it download as fast as Bungie will allow. Next, click over to the "Scan Rules" tab (4). Looks like this. Open the "B.net Exceptions and Campaign History Quick" text file, and copy/paste the first 21 addresses (making sure the "-" symbol is in front of them) into the text box on the "Scan Rules" tab in the options menu. Once you see them in the text box, click "OK" (2) to save and exit.

To find out what URLs we need, we have to go to halo.bungie.net. Once there, select the drop down menu (1), and click the game you want to find your stats in (2). Enter your gamertag in the search bar (3), then click the Marathon logo (4) to search.

For Halo 2

H2 is very simple. In the .txt file "B.net H2 History Quick" replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All)
Now find how many pages worth of H2 you played and copy/paste the correct amount of pages from that document into the URL list of your HTTrack project. If you played enough games to exceed the URL box's character limit, you have to split it into multiple projects.

For Halo 3 Matchmaking

H3 starts off simple enough with Matchmaking. Click the "Halo 3" tab on halo.bungie.net (1), then click the "Matchmaking History" tab (2) and note the amount of pages, as seen here. Now with that number in mind, load up the "B.net H3 MM History Quick" .txt file and replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All). Copy/paste the correct amount of pages from that document into the URL list of your HTTrack project (making sure to include the first 6 links as well). If you played enough games to exceed the URL box's character limit, you have to split it into multiple projects.

For Halo 3 Customs

First Click the "Halo 3" tab on halo.bungie.net (1), then click the "Custom Game History" tab (2). Here's where things get a little messy. The page count may be inaccurate until you find the last page. For example, it might say "83 pages", but if you click on page 83 you get an error. Then if you click on page 77, it says only 77 pages. So What you'll have to do is click through until you find the true last page. Once you've found it, open the "B.net H3 Customs History Quick" .txt file and replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All). Copy/paste the correct amount of pages from that document into the URL list of your HTTrack project. If you played enough games to exceed the URL box's character limit, you have to split it into multiple projects.

For Halo 3 Campaign

Back to simplicity. Click the "Campaign History" tab under "Halo 3" on halo.bungie.net and note the amount of pages. Now with that number in mind, load up the "B.net Exceptions and Campaign History Quick" .txt file and replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All). Copy/paste the correct amount of pages from the second section of that document into the URL list of your HTTrack project.

For ODST

To find how many pages you need to capture for ODST, on halo.bungie.net click the "Halo 3: ODST" tab. Under that select "Campaign History". Note how many pages there are to capture and open the "B.net Exceptions and Campaign History Quick" .txt file and replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All). Copy/paste the correct amount of pages from the third section of that document into the URL list of your HTTrack project.
Now for firefight it's the same process. on halo.bungie.net click the "Halo 3: ODST" tab. Under that select "Firefight History". Note how many pages there are to capture and open the "B.net Exceptions and Campaign History Quick" .txt file and replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All). Copy/paste the correct amount of pages from the fourth section of that document into the URL list of your HTTrack project.

For Reach

Reach can be hard to find out your pages. At your Halo: Reach service record on halo.bungie.net, note how many games it says you've played (1), this will give us a good idea of how many pages there will be. Now click "Game History" on the left (2). What you have to do is find the last page manually. To do this scroll all the way down and click "Next". You could just hit the "Next" button over and over, but it's much quicker to edit the number at the end of the URL. Every page contains 25 games, so take the total amount of games you played, and divide by 25. If you played 1000 games of Reach, you'll want to go to page 40. Or as in our example image, 1,487 games means go to page 59. It isn't always exact, so you may have to change the URL one by one to find the last page. Once you've found the last page, open the "B.net HR History Quick" .txt file and replace all instances of "GAMER+TAG" with your desired gamertag (in Notepad, click "Edit">"Replace...">"Find what: GAMER+TAG>Replace with: Your Tag>Replace All). Copy/paste the correct amount of pages from that document into the URL list of your HTTrack project (making sure to include the first 12 links as well). If you played enough games to exceed the URL box's character limit, you have to split it into multiple projects.

Once you've got all your pages in the URL box of your HTTrack project (or as many as you could fit), click "Next". On the next screen, click "Finish". The program will begin saving your pages. Depending on how much you played, this can take a long time. Hours. Also bungie.net gives errors fairly frequently, so I highly recommend you run the program multiple times to ensure you get all of your data.

Running it again

The second time you run the program will be far easier, because it will save your settings from the first time. But you have to be careful not to overwrite the first copy,
When it finishes the first time, click "Finish". Now from the title, click the down arrow and select the file you want to run again. Click "Next" ONLY ONCE. We have to go move some files around before proceeding.
no to wherever you saved your data to, (the default being This PC>Windows(C:)>C:\My Web Sites) and make a new folder (right click>New>Folder). Give the new folder a name similar to your saved data's. For example, if you named it "B.net GAMERTAG", then name the new folder "B.net GAMERTAG Complete".
Now back to HTTrack, click the "Action" drop down menu and change it from "Update existing download" to "Download website(s)". Click "Next", and then click "Finish".

Backing up your files

Once you have all your files, and however many copies you felt like doing, right click the containing folder and select Send to>Compressed (zipped) folder. That may take a while but once it's done, we're ready to upload it somewhere. There are many places to do this (and you should do as many as you can), but the one I'll focus on is Archive.org.
After creating an account and signing in, click the "UPLOAD" button in the top-right. Now click the green "Upload Files" button. Then click the blue "Choose files to upload" button. Find and select the zip folder you created, then you'll see this screen. Add a description (1), for example "All stats on halo.bungie.net for player GAMERTAG on Feb 1st, 2021.". Next you have to add some tags, separated by commas. I suggest "halo, bungie, bungie.net, stats, GAMERTAG". Thirdly, you'll have to select what collection this upload belongs to (3). Select "Community Data". Now just click "Upload and Create Your Item" (4), wait for the upload to finish and you're done!