Dude, of course I'm not taking the topic's title literally. Bears can't be ninjas cuz they're too big, and they can't be snipers cuz they got no opposable thumbs!
Bears are Mammals, so I don't see the issue there.
But does that make them "Ninjas"? Let's see.
You still need to Fight All the Time. Warlocks have unlimited Eldritch Blast, so they could literally be fighting all the time they aren't sleeping(which ninjas, being Mammals, still have to do, and since that's requirement number one, it beats requirement number 2 on priority). Even if you're blowing up rocks, you're still fighting.
And finally, you have to Flip Out and Kill People. Hmm...i think this calls for Druidic Avenger. Being able to Rage makes us able to Flip Out and Kill People, which will satisfy the final requirement for being a ninja.