Since you're playing on normal, I'd say do your best to minimize losses, but don't let them exceed 25% - if that happens I'd reload and try again. Also be aware of what troops you're losing - low level troops usually are plentiful while higher level troops are much more rare. If you only have a few higher level troops available, any loss there might be unacceptable so I'd say try to do your best to have no loss on your level 4 and 5 troops.
As far as your Hero's stats, the Attack and Defense add directly to your units Attack / Defense. So if you have 3 Attack and 2 Defense, then your units will get +3 to their Attack and +2 to their Defense (base values are in parentheses () when you pull up the unit's card).
Good luck and have fun playing!
/C\/C\
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