You meet pouncers several times before but in act 5-1 there are 2 pouncers directly at the beginning. Weaken one of the two pouncers and wait until he jumps on you. Kill him with the shotgun or chainsaw while he is in the air.
Note from the comments:
I found it much easier to just unload a clip into them then leave the chainsaw running. Much easier than trying to time my shotgun shot perfectly.