Just ask Jeb how well his ad blitzes worked for him.
Jeb's Super PAC never spent big going after Trump, mostly they did pro-Jeb ads and some anti-Rubio and anti-Christie ads. Kasich's Super PAC has also dabbled in anti-Trump ads as has the Club for Growth, but nothing of this size. That being said, spends have to be larger because they are in multiple states now.
As many have noted, the time to spend big to go after Trump was before any voting started. That is what happened to Gingrich in IA back in late 2007 which took him down and then when he came back the outside groups again took him down in Florida.