Ice, when engaged and NOT killed will trigger Floe.
Floe, when engaged with Ice and NOT killed will despawn and trigger Icefloe. Unsure if you have to beat them down to a certain percentage or if it is just enough time lapses, but Icefloe spawned when I had brought both mobs down to ~50%.
Killing Ice after Floe is up will despawn Floe.
Killing Floe will despawn Ice.
Neither mobs die, and hence not killed, when Icefloe spawns.
Meaning, in order to get the hunter achievement associated with this mob you have to wait for Ice to spawn three times. I did not see Ice spawn for well over 500 spawns in the basement.