As a player who uses a dying wish deck I can tell you nether summoning is an amazing card.
How to use it? Once you end your turn, keep track of all minions that die after that point. If the things that die are two big minions, you can get massive value from nether summoning. It really isn’t that random at all, just keep track and weigh the probabilities. Even if you don’t get the minion you were hoping for, summoning at least 5 mana worth of minions is good (which usually happens). If you want to get a specific minion, Reaper of the nine moons is a great card for that, you can place it far away to prevent an unwanted minion from dying, then suicide him into a big threat, like an aymara, now when you use nether summoning you know exactly what you will get.