Here something nice to know for me the Guardian was harder to beat them Gaia the final bad guy. 

Tera Domain has 99 floors so if you talk to these people you will get some part to help you.

#1 Techna-Donna
#2 Angemon
#3 Chief Engineer Maestro
#4 Esteena (your 1st part)
#5 Techna-Donna
#6 Angemon (your 2nd part)
#7 Chief Engineer Maestro
#8 Techna-Donna (your 3rd part)
#9 Chief Engineer Maestro (last part)
For the parts u mite have to talk to the people allot.