No need to go to source edit for your problem....
Go to ur item_db.txt (pre or renewal depending on what you're using) and change this part
4077,Phen_Card,Phen Card,6,20,,10,,,,,,,,136,,,,,{ bonus bNoCastCancel,0; bonus bCastrate,25; },{},{}
into
4077,Phen_Card,Phen Card,6,20,,10,,,,,,,,136,,,,,{ bonus bNoCastCancel2,0; bonus bCastrate,25; },{},{}
answer is from rathena/doc/item_bonus.txt where
bonus bNoCastCancel,n; Prevents casting from being interrupted when hit (does not work in GvG | n is meaningless)
bonus bNoCastCancel2,n; Prevents casting from being interrupted when hit (works even in GvG | n is meaningless)
HOPE it WORks !!