Pasted

In my opinion is easier to cut and convert directly in VitualDub before creating a .webm file, but if someone ever get an error doing that, I'll suggest one fast setting that is somewhat easy to do on MeGUI.
 To do this, you’ll need AviSynth installed in your computer and it’s pretty easy, no complications whatsoever.
1. Open MeGUI, press Ctrl+R, select your video and click on “DirectShowSource”. Press “Play” and search for the scene you want. Note down the number of the first and last frame of the cut. Close.
2. You can resize/crop the video if you wish on the “I/O” tab, on the “Filters” tab check “Source is Anime” and, if you are resizing, I suggest using “Spline64”. 
3. Save. 
4. Go to Tools>AVS Cutter. Select the .avs file and insert the frames you noted down. Click on “Add cuts to script”.
5. After that, Encoder settings>x264: *scratchpad*; click on “Config” on the right.
6. Check “Show Advanced Settings”:
7. Encoding Mode>Const. Quality
8. Quality>16~20.
9. Preset>Slower or Very Slow
10. Tuning>Animation
11. AVC Profile>High Profile
12. AVC Level>Unrestricted/Autoguess
13. Save it and select File Format>MP4. Click on Queue and wait. When finished, use the cut and converted file to create the .webm file on VirtualDub.
 This kind of conversion is good for keeping a nice quality and lowering the final file size of longer cuts with more… action.
 PS: If you use the 10bit x264 you'll get a better result concerning the compression vs quality (MeGUI's default x264 is 8bit). You can download and replace it on MeGUI>Tools>x264. You have to download both x264_x86 and x264_x64 versions though.