Renovation pruning will work well with forsythia, if you cut it all back to ground level then rest assured it will respond, done now you would expect by end of season to see regrowth of between 1 to 2ft, however rototilling is that what we call rotovating over here, if so then I think you may struggle with the thick root systems and stumps that have possibly been formed by self layering, so if this is a problem myself I would do the old fashioned way and grub them out with a a decent sturdy mattock, do this gradually, over the course of a week so you don’t overdo it and give yourself an injury, or if a possible get a few of you in it and you will clear it in next to no time, once done keep it in check over the years with pruning after flowering.

My forsythia was burned up in a fire a few years ago, but now it's all grown back looking better than ever. I don't think T-Rex can kill a forsythia. Read my post and the comments.

Good Night Forsythia
http://www.growsonyou.com/question/show/260154

Here's what it looks like now
http://www.growsonyou.com/photo/slideshow/336202-forsythia/member/bathgate