It was frustrating to watch Nebraska's defense play unphysical, soft (i.e. far off the line of scrimmage) coverage and consistently only rush three or four. Purdue's QB is immobile and their WR's are undersized and pose no deep threat so get after the QB by blitzing and play physical against the WR's (i.e. press/jam/lock up/bump/etc.) at the line of scrimmage to knock them off their timing on short routes.
If you know their QB is immobile and wants to throw from a clean pocket then make the pocket uncomfortable by blitzing and putting pressure on him.
If you know they are throwing short routes you don't have to cover for very long so play physical against their WR's at the line of scrimmage, cutoff or step in front of some of those short routes and look for a pick or pass breakup early in the play knowing their QB is under pressure from the blitz.
Purdue ran 101 offensive plays (54 passes and 47 rushes) and never got called for holding.