He barely edged out a win over Hillary. He's not that great of a candidate.
Give the American public someone decent to vote for, who can stand up to him, and campaign everywhere they should campaign, and he likely loses.
Biden, I'm afraid, would be easy pickings for Trump. He just doesn't have it imo. Warren would be OK, but it's really easy to brand her as too far left. Still, could win if she pivots. Pete, could stumble if he can't attract minority voters but otherwise is solid. And Bernie is similar to Warren in terms of weakness, but could likely do OK.
I've not seen Bloomberg in a debate, although I imagine he'd do fine. I'm sure there's stuff Trump can throw at him, but I agree he's well positioned to be able to handle Trump. I just don't think he has the convictions of office, and I'm not sold on his judgment politically although I could be convinced. I think he's a capable leader who is polling to find out the policies that can win it for him, which isn't necessarily a bad thing...I just don't expect conviction from him on the policies he's advancing. Just that he thinks they're the most reasonable today.
In some ways, being a pragmatist rather than a "believer" may be a good fit for politics today. I think it allows Trump to be a bit more flexible on things as he has no values to stand behind, just whatever gets him the vote. If Bloomberg is a little like that, but somewhat more principled in what he's willing to do (things like, oh, I dunno, following rule of law, not soliciting foreign governments to interfere...you know, that stuff), then it may give him some good wiggle room in negotiations where he's not married to any single position but just wants to advance the ball down the field.