Reduce the number of thralls defending your base. That apparently drastically reduces the number of attackers.

I built a new base which I successfully defended against level 8 purges without any defenders at all, just using gas orbs, a bow and a pike.

Defending my main base - which has 40+ thralls inside - even against a level 3 purge, in contrast, is near impossible. The attackers stagger everything to death, and the game is likely to crash.

So… less is more, apparently.