Is there a minimum combat factor?

To the best of my knowledge your combat factor can go below 0.

However it may be impossible to engineer the fight between two elements that have factors of -3 and -2 so that when each rolls a one you finish up with totals of -2 and -1

Technically -2 is double -1 so the winner of the fight is killed. In effect we can feel safe that such an event is so unlikely that you can allow combat factors to go below 0 and the outcome of the dice rolls ensures that sensible results will generally follow

Cheers

You've hit the nail on the head sheffmark! You cannot double 0! So those Hordes at -1 are (relatively) safe if they can roll a 1. 

Cheers

Jim

It is worth noting that the being doubling the victim is the minimum requirement for the kill

In fact the rules text is
"If its is total half or less than half that of its opponents" --> P11 Para 7 line 1

0/2 = 0 and so 0 is less than half the value of 1

The argument that if 0/2 = 0 means that if both sides finish on a score of 0 then they are both doubled can be countered by the fact that further up the page there is a rule for
"If its total is equal to that of its opponent"
So we're good.

Where we go wrong - from my memory of high school math and my day to day work where I do math for a living

Similarly -2/2 = -1 - so technically the element with a score of -1 is half or less than half of the element with a score of -2 so would be destroyed.

Fortunately it is perilously difficult to engineer a situation where both sides go in with negative combat factors and the odds go up if the situation involves two players who would slavishly hold to the wording and argue that either
0-0 is a double annihilation
-2 vs -1 leads to the higher score (-1) being the casualty.

cheers