(Fogpatrols also increase as above: 13, 18, 23, 28 if fog-event comes back at some point)
The problem with this particular task is that if you have a strength of 3 or more, even if you lose the fight in with the patrol, you lose 1 strength point. With the next attack you then have more percentages as a chance of victory, but THAT is exactly what is not wanted here.<br />
That means you have to find a patrol somewhere (Egypt or Gaul) that has a strength of 20. Don't be fooled by the bug that still exists with ambushes - they are often incorrectly displayed as 20 when they are actually 35, for example (see tiering above).
If you are already too high in the village level, you can also create the 20 Patrol yourself by "carefully" fighting down a stronger patrol.