By not doing that?

If it you're going hours without making any progress, you should be rebirthing. Hell, if you're going *minutes* without making any progress it's probably worth considering it. Progress in this game isn't measured in what stage you get to, it's in building up your moetifacts so you can go further with less effort. And when you hit certain milestones, things escalate quickly - my standard run strategy involves not even hiring any heroes until the one after the toy maker, with all of my attacking other than the first few seconds coming from perma-Phantom Barrage.

If your focus is getting Isabel to L4 for the benefit in AB (and I'm assuming you need to do that within the event period), you should probably rebirth even more rapidly. You get hearts by beating the bosses that appear every five levels, which lets you build them up much faster than if you wait for it to fill up on it's own. It's been long enough since I started playing that I'm not sure what kind of defeat speed you're getting at the early levels, but I'm sure you'll make progress on your actual goal faster well before you actually run into the wall.