I'm making this guide partly so when I want to play this in the future I don't need to re-invent the wheel so to speak but probably by that time this information will be obsolete because of updated software.
There is also a slight chance other people will want to play this game and I also want to share some thoughts about it so be sure to read the bottom part of this post.
I do assume you understand some basic Windows and Dosbox tasks as how mounting folders and drives work, if not check out https://www.dosbox.com/wiki/ and https://dosbox-x.com/wiki/

Step 1.
Download Dosbox-x and dgVoodoo2.
I use the portable win x64 version of Dosbox-x because I have Windows 10 x64 and portable because I find that more convenient than a centralized Dosbox-x install.

https://dosbox-x.com/
http://dege.freeweb.hu/

Step 2.
Uncompress the contents of the Dosb0x-x file you downloaded into a fresh folder, let's call it "extrass" because I think it's a funny name but anything else will do.
From the compressed dgVoodoo2 file extract dgVoodoo.conf and dgVoodooCpl.exe to \extrass\ and go to the compressed 3dfx folder and then to x64 and extract the 3 Glide .dll's again to the \extrass\ folder.

Step 3.
Editing dosbox-x.reference.conf.
Open the aforementioned file with your favourite text editor.
There are many customiztions that can be made but for clarity I will mention only the critical ones here.
Within the [dosbox] section change memsize = 64.
Within the [cpu] section change cputype = pentium_mmx.
Within the [voodoo] section change glide = true and lfb = full.

Step 4.
Rightclick the dosbox-x.exe and select create shortcut.
Now rightclick the newly made shortcut, then select properties and add -conf dosbox-x.reference.conf to the target of the shortcut and click OK.
Doubleclick the shortcut and start Dosbox-x.
Now dust off that Extreme Assault disc and pop the sucker in (AVGN reference 😁 ).
Mount the \extrass\ folder as the C:\ drive and the Extreme Assault CD-rom as the D:\ drive
Go to the D:\ drive (in Dosbox-x) and type install and let the game install and setup the soundcard and graphics.

Step 5.
Patching the game.
Download the patches here https://community.pcgamingwiki.com/files/file … ssault-patches/
Extract and execute the patches for your version in sequence, just put the .exe's in the \bluebyte\extreme\ folder and execute them within Dosbox-x (in sequence!).
Pro gamer tip: use Dosbox-x's turbo mode.
Then from the file explorer rename or delete GLIDE2X.OVL in the \extrass\bluebyte\extreme\ folder.

Step 6.
Setting up dgVoodoo2 by starting dgVoodooCpl.exe.
In the Genera tab change Output API to Direct3D 12 (feature level 12.0) and Scaling mode to Stretched, keep Aspect Ratio.

Few... that's pretty much it, now you can start playing by mounting the \bluebyte\extreme\ folder and the CD-rom if you want to enjoy the soundtrack from Chris Hülsbeck, start Extreme Assault by typing 3dfx or xa_3dfx from within Dosbox-x.

However........., while I enjoy the smooth looking 3dfx graphics, I do not enjoy the sub 30 fps I get on my computer which often tanks below 10 fps.
The solution is to just play the software version, which actually looks pretty great too with the positive benefit of having a higher stable framerate.
Hence the cputype = pentium_mmx (you still need Dosbox-x), also max out the graphics option in the main menu.