You are talking about two different levels.
There is the World War 2320 level (one Boss), and then the "Oil" level, where the Boss makes clones.
They are both definitely playable (I'm not a particularly great Warlight player, and I've tried them both multiple times and won on every attempt, including the first try).
Here's a thread about the first level:
https://www.warlight.net/Forum/154835-having-trouble-world-war-2320-bossFor the one where the Boss clones himself, yes, you must kill all the clones. Each Boss can only clone itself twice, so eventually you will kill them all.
There is also a level where the Boss gets stronger over time. That one can also be challenging, but the good news is that he does "max out" (on turn 15 or so) and doesn't get any stronger after that.
They are challenging but certainly not unbeatable! Good luck.