This is just what worked for me--your millage may vary.

1) Turn off motion blur
Go to C:\Users\username\Documents\My games\DRAGON QUEST XI S\Steam\Saved\Config\WindowsNoEditor
and add this text to the end of Engine.ini

[SystemSettings]
r.MotionBlurQuality=0

[/script/engine.renderersettings]
r.DefaultFeature.MotionBlur=False

2) Play on the normal battle speed.
Yes, this slows the game down a lot, but for me personally, I think the constant flashing lights of spells and abilities was causing my motion sickness.

3) Turn off the automatic player camera tracking
Turning this off makes it so that the camera is not changing as much, as you are unlikely to move it unless it really needs to be moved.

4) Set camera rotation speed to 10
This reduces the time spent with the camera spinning

5) Turn down the brightness
This is self explanatory (I set mine around 30).

6) Set post processing quality to 1.

7) Set anti-aliasing quality to 4.

The above 7 things completely removed motion sickness for me.

The following things may also help:

1) Play with a mouse and keyboard.

2) Turn on a light behind your monitor if there is no light there.

3) Sit further away from the screen.

4) Hydrate and take breaks.

5) Get a desk fan and have it blow on your face.

And lastly, if all else fails, switch to 2D mode.