Yes, ball peen hammers are a no no.....be VERY careful on the 4 eye cars as those dashes can be VERY brittle and break easily.
You need to discharge the A/C to do it....I have done it without discharging the A/C but it's a pain...can be done but you will have skinned up hands and will probably utter a few colorful words in the process.
The more you do, the better you'll get at it.....but it still sucks
While you are there check your evaporator core to make sure it is good...otherwise you'll be doing it all over again for that one