I agree with those mentioning that due to trebs you will always encounter losses.
What I'd suggest (and will be doing myself - as soon as I make the time), is to send a meat shield to take out the trebs and time a 2nd attack to hit w/in seconds with ballistas and transports. This should keep losses to an absolute minimum. In the 2nd attack you should loose nothing.
Basically, by taking out the trebs in the first attack, the 2nd attack will see the l6 NPC as a l5,of course you'll get the resources, medals, prest, experience, etc... of an l6.

