Maybe give them additional time like 15 10 or material advantage. Takeaway one of your minor pieces.
Guidelines for Time Handicap?

By G/15 I guess you mean 15 minute games.
The accepted rule is this: Young guys (aged under 30) get no handicap. ages 31-40 get 2 minutes added; 41-50 get 5 minutes added, >50 get ten minutes.
The boss gets no handicap at all because he might find that demeaning, so he plays with the stock-standard 15 minutes .... BUT YOU MAKE SURE HE WINS!
Hope that helps.
We plan to play some G/15 at the office but the playing strength differs. Is there any usefull guidelines for Time Handicap?