Lord Marrowgar is a great introduction to hard modes. It is twitchy and confusing and simply great!
There are several changes to the encounter. The most important one is that now bonespike happens all the time, so during the whirlwind phase, yes, RNG can happen so you are spiked, fired and spun upon all at the same time. There is not a lot you can do about that sometimes.
So, phase1 is business as usual. Marrowgar, of course, hits harder.
Phase 2 is where things get fun. Since people can be bonespiked, it is imperative that you get them out asap. Also, the whirlwind thing hits a lot harder, so you cant plant your feet and heal youself through it.
So, you divide the raid up into 2 groups, one goes left one goes right. Spread out a bit, but make sure you are in range of heals. The idea is to ping pong marrowgar left and right between the two groups. Try and avoid the blue fire on the ground, and as soon as you see Marrowgar heading in your direction, run away. I cannot emphasize enough how important it is to move as SOON as you see marrowgar swap targets to someone on your side. However, keep the group together, so having 2 designated spots to run between on your side can help with this. If anyone on your side gets spiked, get them out as soon as you can, and healers, try to keep them alive.
But really, that's it! Heal hard, get those people out of the bone spikes as soon as possible, and ping pong Marrowgar between the raid groups, while staying away from his spinning blades of death, and not standing in fires.