Most of it will depend on the team comps and map, as well as what role you are playing. One typical way is to find one or two of the enemy team alone or overextended, such as trying to steal one of your camps, and kill them, rather than going for full 5v5 team fight. The numbers advantage will definitely counter their level advantage. One of the comebacks built into the design of the game itself is that the team with the lower level will get a lot more experience for a kill. Often just a few kills will completely catch you up.
You can also set an ambush up, although that can be riskier, but often the value of surprise will also lead to a key pick.
Soaking is OK, but won’t overcome the lead on its own. Soaking is the best route if you are close to a talent tier, or have heroes that directly benefit from it, such as an Azmodan or Nazeebo.
And last but not least, you can look at your team comp and its particular strengths, and abuse the heck out of it. In the screen shot I linked, we had a unique opportunity to pull of a completely unexpected and unorthodox strategy, very similar to a Juice Pirates. Because of the way the map is laid out, we had a chance to attack the bottom lane by skipping tower defenses, and using Medivh portals to get directly to the fort, keep, and eventually, core. Anub’arak and Gazlowe can call up summons to tank fort shots for us, and Greymane and Artanis are excellent at ripping through structures. Yes, it was a bit of a cheesy thing to do, but at the end of the day, victory or defeat is determined not by levels, not by the number of kills, but by whose core lies in ruins.
Which heroes do you play, and is it on specific maps you have issues? One thing you will learn if you hang around enough, people on this forum love to give advice!
(Of course, it may not always be good advice, but we do try…)