The instant kill is actually instant battle where you are assumed to spam basic attack.
It is actually possible to die during such a thing, which I have managed to actually do once after going through like 50 of them without rest. Because I assumed they just get auto killed without costing me anything.
Also you don't want iron armor, it is actually a bad tradeoff. As I explain here
https://f95zone.to/threads/a-struggle-with-sin-v0-4-7-6a-chyos.32136/post-7640774
each piece of iron armor +2 gives you -3 evasion +2 defense compared to leather armor +2
For a total of -12 evasion and +8 def. A bad tradeoff that actually weakens you, despite being much more expensive and requiring a ridiculous amount of talent points invested into armor talent