The Cruader isn't immune to the Slagstorms. It's not being targeted by it, so it will still take the damage.
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You could use Wrexial to put the final damage in. I have a Wrexial EDH deck myself, and I run Urborg, tomb of Yawgmoth. It's amazing with hime and it puts a four or five turn timer on the game. If you could get an Urborg, it would greatly help that deck. A little more land in general will help you as well.
While playing infect in EDH didn't warrent that girl's response, the reason people despise it so much is because you only have to deal 10 damage to someone while they all have to do 40. That's only a quarter of the damage. So unless they're playing a control type deck, you're going to win before the game even gets started. That's why everyone hates infect EDH.
Don't run Nissa Revane if you have only five elves it's just not worth it.
If you're gonna run Niv, at least run a curiosity as a very good win condition.