Despite the fact that every other guide seems to state something to the contrary, you CAN and WILL complete this achievement (and the Taunt 50 Gang Members Challenge) easiest by repeatedly taunting DURING combat WITH notoriety in full effect.
As of five minutes ago (as of this writing), I can confirm that this works, even if you've already achieved 100% city takeover. If you're looking to complete those two items (the achievement and the challenge) at the same time, simply start an activity like Snatch (something where you're on the ground and will be chased by gang members in cars). Stir up quite a bit of trouble and then quit the activity but DON'T WIPE YOUR NOTORIETY.
So long as you have immunity to bullet and explosion damage, you can then walk right up to gang members who are actively trying to kill you and press the taunt button to see the "Taunting" Repsect charger appear in the top right of your screen. The gang member may not verbally or even physically respond to your action of taunting them (since they're too busy firing their guns at you), but the game WILL register this as counting, and you'll see your progress show up on the ticker after every 10 taunts or so.
Again, this can and does work while in the middle of a firefight, but the same rules apply as elsewhere: you can only taunt a single gang member once to count for both the challenge and the achievement, and you may need to stand right in front of their face (hence the need for bullet damage immunity) and tap the button multiple times for it to actually take effect. Once you've wiped out your currently selection of gang members, pull out a weapon (preferably a rocket launcher with infinite ammo), clear them all out and wait for 5 more cars to show up, full of additional gang members you can taunt.
This is in no way as complicated as many others have made it seem. Just make sure you're immune to damage and you'll complete both the achievement and the taunting challenge in less than 15 minutes.
Edit: I made a video for proof.