You might want to be a bit more specific on the YST unkillable team.  I know that YST is a content creator, which is who I assume you mean, but I believe he has done a number of unkillable teams.  Can you link to the one that you are looking to replicate?

https://youtu.be/FJnys-iN2-0

this is the one 

Can you confirm if any of the champions has lore of steel mastery?

I ran the above numbers through the CB calculator (https://deadwoodjedi.info/cb) and I think the issue is the fast Helicath with the 3x Speed gear.  At 277 speed in 3x speed gear, he actually has 276.71 speed, which breaks the speed tune in the calculator.

Adding +1 speed to him would take him to 278 listed speed, or 277.71 calculated speed, which re-aligns the tune on UMN.

This is just a high level guess though as I don't know the masteries of each of the champions

Edit: Just want to add that if you want to explore this +1 speed change then plan accordingly, because if you glyph up an item to add the +1 speed, you cannot remove it again later.  It is better to find another piece of speed gear that you can use to get the +1 speed on him, so that you can always revert back to the original gear if you want to test further.

Ninja, slow heli and fatman have lore of steel.

I can try to give him a +1

I already want to thank you for your help it's very kind of you ! 

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You were totally right with adding +1 speed !! 

you are amazing thank you so much!

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Great to hear that this fixed it!  I did confirm before that lore of steel on those other 3 champions wouldn't have had an impact, so the +1 speed on the fast Helicath would have done the trick.