An idea (or two) strikes me...
Issue: this week's event incentivises people to stick Humans and Darkstone teams on defence - yes I am seeing more of them, or at least much more than the zero I would normally - however this works against PvP players who need to be out killing Undead troops this week.
(separate question of whether this an issue or not, or whether we are driving people to choose PvP or other modes - just bear with me for now)
1: simply make the Progress (killing stuff for Snotstones) event tie to the weekly theme events
and / or:
2: incentivise players to put out a matching PvP defence by allowing them to accrue Snotstones if their themed defence is invaded. e.g. gain 1 Snotstone for each defence loss with a matching team and 2 for each defence win... Or encourage them to have matching themed troops on defence by allowing: for each 4 Snotstones earned by players invading you, you earn 1 free Snotstone...