Immunity is an ability that often gets overlooked in battles. I think that part of the reason is because many cards with the ability don't have an attack. Kralus and The Forgotten one used to be the only ones that have an attack straight from level one. But now there are lots more.
Here's a list of cards on the market with Immunity. There's also Evelyn Auvera which is a soulbound card with the ability. As you can see, it's a pretty pricey ability to have apart from the Forgotten One or the Blackmoor Nymph.
Combining Immunity with Weapons Training
Where I think Immunity excels is when you combine it with a Weapons Trainer. Take a look at this battle where Poison was part of the rules. To maximize my chances of success, I decided to include as many cards with Immunity. The problem is that two of them - Almo Cambio and Evelyn Auvera don't have an attack.
Without Weapons Training, Those two would just be adding health to the lineup, hoping that the Poison kills the opponent's units fast enough. But that's not really a viable strategy these days, with opponents becoming smarter.
So, with a Weapons Trainer, I was able to give them 2 magic attack each, as well as immunity.
As you can see, my opponent's units were still affected by Poison, which meant that they took an additional two damage per round, while my immune units were unaffected. So just getting them down to two health is enough for the poison to knock them out.
Eventually, it was a win for me, despite my opponent using some heavy hitters that did a lot of damage and had high health as well. You can catch a replay of the entire battle here: https://splinterlands.com?p=battle&id=sl_2260ef75171c269901935a26e1724053&ref=charcoalbuffet
So, I hope you consider getting some cards with Immunity in your deck and also combine them with a Weapons Trainer so that they can really do some damage while being immune from Poison and other maladies.