PK Thunder is a spell used by Kumatora. It has the power to break through PSI Shields. PK Thunder comes in four forms: PK Thunder α, PK Thunder β, PK Thunder γ, and PK Thunder Ω. Each new form increases not only the power, but the amount of bolts that strike the enemy. For example, PK Thunder α strikes with only one bolt. PK Thunder Ω, however, strikes with four. The enemy the bolts hit is random. You cannot choose which enemy the bolts strike.
It may not be a Thunder based PSI but PK Ground is believed to be the 5th level of PK Thunder.