I've had success in the mornings chasing them down and waiting until they get quiet and start to bed down, then barging in on the herd and intentionally blowing some cows out and then bugling at them. The herd bull WILL come see who's harrassing his cows. Is that agressive enough?
Need to do this with two people. One person to do the calling and the shooter to keep sneeking. I can't say enough about how critical it is to know where he was the last time he bugled, usually he'll still be close to that spot.
This basic method even worked one time when a herd decided to bed on both sides of a road and we drove a 4-wheeler right into them in the middle of the day, not knowing they were there. Some cows went one way and some went the other. I whipped out my bugle and my partner hopped of, grabbed his bow and the bull came running out into the road 40 yards in front of him. He shot & missed (had a bite & blow cow call in his mouth when he came to full draw, spit it out and it wrapped around his string, nearly took his head off when he let the arrow fly!! I kept calling and beleive it or not the bull came back and let let my partner have a 2nd chance. He was so shook up that he missed again!! I'll never let him live that one down!
The bottom line is what was said earlier, most people are not agressive enough. Elk will put up with more than you think... as long as they don't smell you!! Once they smell you, it's over.