I believe it's recommended for VR5 since that is the gear level it drops. A group of Vr1-3 will probably have a tough time with it. It is certainly possible as I cleared it with VR4's a few days ago.
I killed Imiril with two v1's and two v2's back in April.
Granted, it took a looooong time to down him, but man... that sense of accomplishment. I'll also agree that it isn't tuned to the same level of difficulty as any other v5 fight, or to be honest, most v10 fights... but it's totally doable.
With a bit of CC and AoE damage Keeper Imirin is easy. I've downed her many times From VR1 to VR5 without issue. Save AoE ults for that wave, make sure you can keep them together.
Save your ulimates for the wave that spawns clanfears (wave 4) and if your dps is slow, wave 8, if you get to wave 12, yet more clanfears, your DPS is too low and give up.
Just do not let the Clanfears stay up, and a good AOE ulimate will take care of that for you.