Just spit-balling here, but I believe it increases by 0.5 per zombie killed. The relevant parts seem to be:
Code:
<Row>
<ModifierId>TOWERDEFENSE_INCREMENT_ZOMBIE_DEATHS</ModifierId>
<Name>Amount</Name>
<Value>1</Value>
</Row>
This is part of a counter that keeps track of the zombie deaths.
Code:
<Row>
<ModifierId>TOWERDEFENSE_ZOMBIE_COMBAT_STRENGTH_FROM_PROPERTY</ModifierId>
<Name>Scalar</Name>
<Value>.5</Value>
</Row>
And, this multiplies the previous value by 0.5.
There's also these two arguments, which I'm not sure precisely what they do, since I haven't been able to play around with them yet.
Code:
<Row>
<ModifierId>TOWERDEFENSE_ZOMBIE_COMBAT_STRENGTH_FROM_PROPERTY</ModifierId>
<Name>InversePlayerScaling</Name>
<Value>True</Value>
</Row>
<Row>
<ModifierId>TOWERDEFENSE_ZOMBIE_COMBAT_STRENGTH_FROM_PROPERTY</ModifierId>
<Name>AdvancedStartMultiplier</Name>
<Value>50</Value>
</Row>