Hi i am in the process of changing my radiator and thermostat and I managed to get all 4 of those small pesky clips holding in the cooler lines to the rad, but only one of the lines slid out.
Im scared to pry too hard on the lines as they are steel i was thinking of making a wooden wedge and hit it in to seperate it. i am out of ideas, damn canadian winters use so much salt i regret working on my own vehicle nothing ever goes right

 

i have had trouble with mine coming out as well. just apply some force and be sure not to force it where there is a bend in the line. or you could go with the tapping method. they hang in there pretty tight.

 

buddy came over and we managed to get them out with adjustable wrench and prying. they are so corroded i think it would be wist to replace my oil cooler lines. My buddy was just over he advised me to just change them while im in there his wife blew his blazers motor by the lines leaking. Go gonna change the cooler line assembly as well have all new stuff so i dont have to dick around with these little clips again lol

 

Sounds like a plan...no sense in doing the job twice.
good luck..hope all goes smoothly for ya