The short answer is if you have people getting smoked in T1 that quickly, they need to boost their survivability and/or you need to have a traited LM making sure the grim is debuffed.
I've never noticed much difference between the times when a tank has aggro on the grim and when a ranged has it. Everyone at a certain range is liable to be targeted anyway.
Maybe on that particular run you had bad luck with squishies being targeted for multiple attacks in a short time frame. It happens. I've been on a few T2/C runs recently and haven't noticed any change in mechanics.
[center][color=white][b]DROVER[/b] - champion[/color][/center]
[center][color=white][b]DROGARTH[/b] - captain[/color] [color=red][b]DROVANN[/b] - guardian[/color] [color=white][b]DROTHIR[/b] - Loremaster[/color][/center]
[center][color=white][b]DROVII[/b] - hunter[/color] [color=red][b]DROGREN[/b] - burglar[/color][/center]