How am I supposed to not get outsmited when the enemy jungler has more levels than me

They stole my heart.

Coordinate with your team to burst it or use a damaging ability at the same time you smite. Alternatively, have your team help you zone away the enemy jungler.

Like what sorcerer said, just know your damage output and put as much damage on it to set it to 500 hp then smite it! :) sometimes you just can't outsmite, especially without your team. :(

Vision....really all about vision. If you don't have vision on their enemy jungler...don't do baron. Tease them out with the baron...but let your team know it's a bait and to not stay on it when the enemy shows up. You can't ever get into a 50/50 smite war with another jungler if you're down levels...unless of course you have a Kalista on your team.

There are possible exceptions...like Lee Sin. You can Q smite with Lee Sin and do considerable more damage than a smite alone since his Q does damage based off missing health.

Ward the area, if he comes peel off of dragon and fight him, even better if your teammates do it for you and you can stay on dragon but you can't always depend on your teammates to do that kind of stuff.

Peel him off, or use another ability paired with your smite. Cho'Gaths feast, Nunu's Consume, or even a small one stacked with your smite like Rek'Sai's E which deals true damage so it is easier to calculate.

kill the enemy jungler?