Even with a longneck rifle, remember to lead by a LOT when it's flying horizontally past you. Small hitbox + lag = some arrows and darts registering hits on your screen, but not on the server. At 30ms lag it's about a body length inc. tail.
Bolas really are the best plan, to get them to SIT. THE HECK. DOWN. You need to lead more because of the slow travel time for bolas, but getting the dimo down is such a relief.
Aggroing headshots also works, but can do more damage than you intend. Make sure to bring a lot of bolas, and maybe clear the dimo's flight path of other dangers first so you don't, y'know, run directly into an alpha raptor while staring at the sky...
Ghillie suits are great if you're trying to snag one in a dangerous area! Good luck building your death swarm.