We were able to successfully print from an Apple IIc Plus to a LaserWriter IINT using the following method.
1. Copied the software (AppleWriter 2.1, in our case) to a 3.5-inch disk.
2. Set up the LaserWriter IINT and set the switches to Diablo mode (SW1 DOWN, SW2 UP)
3. Connected to the Apple IIc Plus with an Apple IIe Printer-8 cable (Mini circ-8-to-DB-25 cable) and a Modem eliminator (DB-25 Male-to-DB-25 Female).
Note: If you have an Apple IIe Modem-8 cable, it will also work correctly.
4. Started up the software and set the parameters on the Apple IIc Plus serial port #1 to 9600 baud, 7 data bits, No parity and 2 stop bits. (Setting the port is done by using the Control-O menu, Option-J "Setting the Printer/Modem
Interface", and entering 9600,7,n,2, Return.)
5. Loaded a file into AppleWriter and press Control-P, then np, to print.
The main reason you may have had difficulty printing with this particular hardware/software configuration -- given correct cabling -- is that you may have assumed the Apple IIc Plus serial ports default to a usable baud rate, as have previous Apple IIc systems.
When setting the Printer/Modem interface through AppleWriter, we noticed that the ports default settings are 0 baud, 8 data bits, no parity, and 2 stop bits. This needs to be changed to the configuration listed above before you can print.