A dremel is a rotory took that has different attachments. one of the attachments is a cutting disk. you can use it to cut through stuff. It spins the attachment at a high rate of speed. can pick one up at lowes or home depot for like $50. also your going to need to drill the wholes to put the new spoiler on. after that you are going to need to use fine sand paper to smooth the wholes you drilled, and then spray the wholes with primer to ensure the wholes dont rust.
Ive not had to do this, however, another option might be to drill through the underside of the trunk upwards. that way you wouldnt need to cut the nipples off.