Help me build an optimised character. (For a good reason)

Me and my family are playing a game of dnd whare it's basically a big test for our dm on how to run combat encounters and how to make dungeons. Becuse of this his goal is to kill all of us in a fair way, we all have three back up character's and where told to make character's we wouldn't miss. As well as to make them as strong as we could. That's why I have come hear to ask you. However can I build the strongest character that I can?

Our character's are level seven with the ability to level up.