Build towers to the left and right of the BuffBeam, making sure they receive red sparks (so they get buffed). With a 4DU BB, 6 towers can be boosted on the left and right; with a little practice, 7 can be boosted (right at the tip, beginning, or end of the BB).
Spore Tower doesn't necessarily need to be buffed by BuffBeam; the Armor Reduction effect isn't increased by BB - only the DoT is.