The HP Based Damage of the Spikes does not kill Rocksters. I have Rocksters with health of 1.63k and Spikes with % of HP at 34%
Surely three hits from a Spike at 34% should kill any monster (3 x 34% = 102%)?
To test this I have removed the Gunshots and Laserguns and left just a row of 11 x Spikes. The monster strength has indeed gone right down after being Spiked four times, but despite having 11 x Spikes and the monsters being spiked over and over again many times, they do not die. I suspect they are eventually killed by the Spikes Health Damage.