I got to 1223. I could have gone further, but I got bored and pressed the "bring next wave" button like 100 times and got pwned. It's not so hard once you figure out how the combonly things work.
Basically, they make combo towers. Say you want to make a nuke (missile + 2 cannons). You place the combonly where you want your nuke tower to be. Then you place a missile and 2 cannons next to it, and upgrade them all to max damage. The combonly will then start flashing between red and blue. When it's on a red flash, click it (the colour defines which particular combo you will make, so if you click it when it;s blue, it will make a different combo). The towers around it will merge into the combonly and you'll have a nuke tower. This tower will only fire nukes, nothing else.
Basically, combonly towers are space saving tools. Without them, you combo group would take up several tower spaces. With the combonly, they only take one.