I'm looking to cut my playing time back to 15 mins a day.

Which aspect of the game should I work first before the time runs out?

I could; go for sending troops out to farm resources, sending resources to banks or stronghold, killing invader and ghosts or train troops.

I'm not sure but I was thinking of training troops.

What to you think?

Firstly remember that the amount of  resources your town can produce will stop once the maximum is reached so always look to use it or send to a bank. 

Be aware of current events,  for example  if there is a resource yielding event then send out troops and maybe then start another task,  before you log off start troop training and leave that to complete or until you next log in.