Cards like the lord of CHange and Fabius Bile say that you choose a player to help you with the battle. Then if you manage to defeat the threat card with help you get 1 completed mission (lord of change) or 1 level (fabius Bile). There are other cards like this as well. Now what happens if you defeat him without any help? do you get the reward mentioned AND get to keep him as a trophy? (if you get help you cant keep him as a trophy). There's nothing on the cards that explain what you get if you defeat them solo.

If you defeat one of these "solo", you keep it as a regular trophy.

TK

No, the cards specifically state "if you defeated it with help". The reason is they're worth 2 levels minimum each as a trophy and thus worth more than either with help.