You don't appear to get stacks for killing frozen enemies, so scratch that ailment.

But for these 7:

Weakness, Curse, Burn, Shock, Poison, Rend, Stasis.

To get all 7 stacks, you need to inflict all 7 of those on enemies, and kill them while the ailment is still on them to get the buffs. Mix up the elements/ailments by using a variety of attacks to get the most out of it.

How is the damage calculated from immortal offering? ie is it just the equivalent of a 25% modifier on gear or is it something else?

i was planning on skipping it as it seemed a rather weak skill, but i was assuming it was just 25% increased damage added into a pool of several hundred % damage already.

Its actually a 25% More multiplier per ailment which is what makes the node so crazy.

Keep in mind though, it adds the 25% More damage as the appropriate element so It can sometimes change which ailments your skills apply, getting all 7 is therefore very tricky.

But even with 3, you almost double your damage, and 5 isn't so hard to reach. good luck!