It will not be very realistic. If the Roman legion is going in Gaul, then it may be surrounded by areas of barbarian tribes, but the Romans are not afraid of this.
Or the same thing happened with the European armies on other continents in the colonial period.
So what I suggest:
1) If the troops are surrounded, then their forces are reduced by 70%, and they have only 30% of their normal strength. Therefore, if they are 10 times stronger, they will become 3 times stronger.
2) And the likelihood that they can move is only 50% - just to reduce their speed.