As the cactus ran towards the group, Isaac ran ahead, drawing the larger one of his 2 knives.
"Cactus incoming!" Isaac yelled.

Isaac sliced at the cactus, wary of its spikes [rolled 2]. The knife was large enough to put distance between Isaac's hand and the cactus spikes, but it was too short to do great harm. Regardless, his knife drew blood, or whatever a cactus had for blood. It was clear that he hadn't done much, however. 

Isaac jumped back, hopefully out of reach of the now living cactus.