This should be a link to his official stat block, but it's not showing that on my end so I'm not certain. I can screenshot if necessary though.
To kill Vecna at level 8 max I either have to go first and one-shot him or have more than 100hp so he can't power word kill me.
I'm gonna assume an open field with nothing around so I can't cheese by saying like "A mountain falls on him. He dies."
I'm also assuming that he's not just going to run from the fight and will fight until either I'm dead or he's dead.
I'm gonna go neither route because I can, infinite damage win!
Placing him at 384 health which is max possible and taking into consideration his resistances and immunities.
I'm limiting myself in magic items to 1 rare and unlimited uncommons and commons.
Because otherwise this isn't a challenge and I do want a challenge.
2 levels of War Wizard to add my Intelligence modifier to my initiative rolls.
3 levels of Gloom Stalker Ranger to add my wisdom to my initiative rolls.
3 Levels of Rogue Swashbuckler to add my Charisma Modifier to my initiative rolls.
That's my 8 levels and gives me a base initiative of 17 if we assume my dex is 20 and those 3 are 18. Obviously that's subject to change depending on rolls, but fuck it that's how we roll.
Variant Human with Alert feat for a +5 to my initiative.
So on a nat1 my initiative is 24 which is higher than he can roll with a nat20.
Ring of spell storing for my infinite familiar glitch.
Unlimited Elemental Gem, Red Corundum's
To explain basically familiars count as creatures and as such can attune to the Ring of Spell Storing allowing them to cast the Find Familiar spell. Because it's a chain effect I can control all of the familiars.
On my turn I shout now to the familiars and then put my Bag of Holding into my other Bag of Holding and vanish to the astral plane so I can't get hit by anything.
My familiars all roll their own initiative and each summon an Fire Elemental. Those fire Elementals take turns attacking Vecna until he's eventually overwhelmed like 6 hrs later.
Honestly killing anything at any level isn't specifically difficult.
