After struggling with healer mana a couple times, we really need to time our channeled potions to a slice where the raid is fairly stable. The boss won't do his line attack while he's slicing so there's no chance we will have to cut them short.
Also, should probably not stand directly inside of the boss for no reason. >.>
In order maximize DPS, it is important to minimize the amount of time needed to be away from the boss avoiding mechanics. On every fight there are ways you can get better DPS by finding the best ways to avoid the mechanics while in DPS range of the Boss/adds.
When Gruul casts shatter on you, *IF* your group is not due for another Inferno Slice soon, you can spread loosely around the melee area in front of the stack point. Make sure it is at least a loose spread, and you arent near other melee while doing this. Shatter will reck you if you get hit by two shatters.
During the rampage phase, spread out. This is less for increasing our dps, and increasing overall raid dps/heals. Spreading out will increase Gruul's potential target areas for his ground smash. This will allow casters to heal and deeps more without having to move and won't hurt our dps at all.
Lastly, if you have a defensive cd as a baseline ability or a talent, make sure you are using it. Use it right before the Inferno Slice is cast (As the tank starts to move over to your stack**This can be seen in video when I hit Divine Protection right before every slice***) This will ensure that despite lag/bad reaction time, you will have it up when the Inferno Slice dmg hits you. This could save your life in case of a bad stack, and at the very least will save healer mana.
So I couldn't tell what your group died to from my POV, but it looks like you guys got the tank debuff right before you got sliced.
Looked like Rabhs got knocked up by one of the lines that didn't look like it should've hit him.
I think some people have the offtank stand in the group that's getting the next slice and taunt right before. Might be worth trying so that flukes like that don't happen and we can re position the groups around the void zones easier.