OlliOlli World

As you progress through the game and learn things such as grinding and manualling, and once you start completing challenges / beating Local Heroes, you will likely have the ability to full combo a level.

However, if you are having difficulties, I unlocked this in a rather cheezy way. On any of the beginning tutorials that are just flat start to finish, once you get close to the finish, just do a manual over the finish line and it should count as you have not done a combo up to that point, and your combo technically lasted the whole level.

This can be done pretty easily on the Old Chippington Mill level in Sketchside. The level has you connecting all 3 grinds by manuals so you will basically get this achievement simply by beating that level. The level is optional so you might miss it, here's a video showing it done:

I’m leaving this here to let everyone know I think you can cheese this achievement, I just got it playing on the first level (before I have manual or grinding unlocked) and just done the level with as many tricks and A taps as possible and it cheevo popped, so not sure if this was just pure luck or if it’s actually possible, just thought I’d let anyone know who wants to try get it early on