Flying helicopters I've never seen anything so severe, but the correction has been the same.
Reduce pressure on the controls, severity of manoevre, power applied. If that doesn't work/not possible, then a hard pull can help.
I have seen ground resonance in a tandem get out of hand, and then quickly controlled with an arm holding the stick tight and same arm locked hard onto the seat. This tells me that the ground resonance was exacerbated by pilot inputs, so a frozen stick may work.
With an older SAS, I've seen PIO that was most easily controlled by turning off the affected system, although identifying the bad one can be a challenge.
Matthew.