I used a Fireball for that Grenade Task. I used a Concussion Grenade and it works. I used a Pineapple Demo Grenade. I even used my M79. The Specialized M79 WILL NOT WORK for the Grenade Task. Going forward, the devs have made it clear that THE ONLY PERMISSIBLE GRENADES FOR THE GLOBAL EVENT are bona fide concussion grenades. It's really difficult, but my hints are this in ANY global event: You must have good aiming. Throw the kink when it's red. If it's orange, you'll miss. If it's red, it's guaranteed to hit. With correct frame input, you can wipe out as many as 4 red enemies at once. THIS IS GUARANTEED IF THE GLOBAL SETTING IS SET TO NORMAL, AND YOU HAVE THE GLOBAL EVENT TURNED ON.
I do all the events and manhunts in the same world difficulty that I am in, namely, Heroic. I think dropping the setting to normal like that is a tad cheesy. I never reset certain mission parts over and over again either as I find that funky too. I do them organically during gameplay in the open world or in missions while bearing in mind the required goals. I managed to get through 90% of this weeks event in one session so it's not overly difficult. The grenade one, the bounties and a couple of others are all that's left but I have the exotic cache so not much incentive to continue on with it to be honest.
i have been doing the current GE event on Challenging.. with 4 directives on...
but usually i run manhunts on normal .. since i do it 4 times.. across all my agents..
it's pretty repetitive.. so... no point in the challenge of it.. and the audio collectibles ain't working nodda this time around.. so there literally is no point.. but i do it anyhow..
if you want quick and dirty.. normal is the way to go... but if you actual find some joy in it.. then sure.. do the upper difficulties.
Throwing grenades isn't hard at all, some veterans or elites may dodge them but red ones (that are grouped) usually don't. Sometimes one miraculously escapes but guess that is when game doesn't want you kill them all to easily. Won't be first time I see this happen, and they are teleported behind cover.
Step 1: Sneak up on them, without alerting any.
Step 2: wait till they are close enough, as many as possible.
Step 3: throw grenade so it covers most if not all NPCs.
Step 4: fire few shots so they react to it and not run away.
Finally the grenade explodes and most of them are killed. When any get away shoot, or worst case you have to run away yourself.