Couple of questions

  1. When I invade, my troops stats are higher than their base stats. Where does the bonus come from, how does it work?

Please check this guide.

  1. When using the siren’s ability on a multi coloured troop, how do I know what colour will be generated? Is it random?

Random.

  1. The difficulty of this game seems really … easy.

Wait for the next patch.

  1. Are the challenges there just to provide a ‘guaranteed’ way of generating souls?

Or for people like me who are tired of invading the same line-ups over and over again and are looking for something different.