How do I come back after falling behind in levels?

First and foremost, always try to be aware of the times that you're not getting exp. You need to be within 1300 range of a dying creep to get the experience, which is about the same distance as a pudge hook or a blink dagger range. Play a few games with the sole target of getting as much exp as possible, so not wandering between lanes too much - use scrolls, and make the most of every creep wave.

If you're playing mid - make sure that you push the wave into the enemy tower before looking for runes, this forces the enemy to defend or lose 3-4 creeps of exp while you have time to pick up a rune without losing any. If you check for a rune (or worse, go back to base) and are missing exp on the mid lane you're putting yourself in a huge disadvantage. On those level based heroes it is critical that you get as much exp as possible. Learn to mind game your opponent in terms of last hitting, to put them off when they try to deny (or better harass them off the lane).

If you're playing a safe lane supportive role, make sure that any creep pulls you do are double stacks, or pull throughs to a 2nd camp - not just single stacks. Single stacks cause the lanes to push to the enemy tower, meaning you have to hang further back from the creep wave to not risk your life, and this makes it much easier for the enemy to zone you out of the experience range.

Always carry a tp scroll from as early as possible - teleporting between lanes rather than walking is 100% worth it if you are missing out on a lot of XP. Ideally, teleport in when the enemy tries to gank one of your lanes, and use your burst damage potential to turn the ganks around - this will give you huge boosts in xp.

Also needless to say, avoid deaths as much as possible. Learn all the best ward spots and keep as good map coverage. The best way to not get ganked is to not be there - and ideally make your enemy waste time rotating between lanes as well.

Hope these general comments are of some use to you, if you want to post a replay ID or two, I'd be happy to give more specific advice. Cheers.

If the other team is destroying you there are really two options to come back:

Team Fights:

Group up with your team, no matter how crappy they are you should still be able to surprise and kill a couple enemy heroes that are alone. Ways to do this could be warding spots they go to alone (jungles) or just catching them in a lane.

This obviously won't work if they're together and stronger than you.

Jungling:

Your team sucking and not doing anything? Just jungle excess time away. Be sure to ward or else the other team will just kill you.