Since theres no more dlc what are the 3 most useful battles that you can use the frost giant summon for? Safe to assume ebony warrior is one. At this point no use saving the giant for later.

IMO if you are strong enough to get the Ice giant, you wont need the reward to summon him :/

The one exception being ebony warrior, as you said.

If you haven't defeated Alduin yet, you could use it for him...possibly.

I loved the battle with Karstaag. I have never felt pain so good.