iPhone 7 Plus Battery Life draining?
How many cycles do you have on the battery? If you have lost >20% of your 
full charge less than ~200 cycles in, your phone is defective and eligible for service.

Failing that, you can go to the Battery tab of Settings and see which apps drain your battery the most. Quit any ones you don't need by double-clicking the home button and swiping up on them.

You can always enable Low-power mode all the time if necessary.

P.S. I have a 7+, and I get a full day (8+ hrs) between Clash Royale, texting, and the occasional web page.