MF Ult overpowered??

I didn't because the example given has no crit items, and reasonably, she shouldn't have any crit items by level 6. She probably shouldn't even have a B.F. Sword by level 6.

The death recap is known to be unreliable, but more likely is that she wasn't level 6, and you are exaggerating. Let's say she got a Pickaxe and a Cloak of Agility, which together costs more than a B. F. Sword, which itself normally costs too much for her to get so early.

Now having 90 AD instead of 105, the total damage of her ult goes down to 810 before armor. However, with 120% damage to account for her crit chance, the total damage before armor becomes 972. Again pretending the Tristana has no armor runes, Doran's blade or any other defensive items, her ult calculated after penetration runes would deal 797 damage. Thunderlord's at level 6 with 90 AD will do 67 damage after magic resist, only four less than with B.F. Sword.

Amusingly, it will do 864 damage total, still just barely leaving Tristana alive.

I mean, sure, other masteries will probably finish Tristana off, but even having a B.F. Sword, much less a Pickaxe and Cloak that early is unreasonable, and even more unreasonable is Tristana having no armor runes. Either way, Miss Fortune's ult is a damage ult that goes on over three seconds. She has to stay still while she uses it and it can be walked, dashed, rolled, jumped or flashed out of, or otherwise interrupted with some kind of cc relatively easily. I don't think it's overpowered in the slightest, and if you've ever played Miss Fortune you might find that most of the time using it is a bad idea. It takes "a lot" of positioning to be able to use it safely, or while expecting to not get interrupted, and even if all goes well it stops you from chasing them, which can let them get away.

I mean, there's a lot of things that are overpowered in the "right circumstances". Take Annie for example. 「 TOTAL MAGIC DAMAGE: 1075 / 1650 / 2225 (+ 980% AP) 」 Copy+Pasted from the wiki, that's the total damage her ult can deal. In order for that to even be possible however, she needs to use it on someone without boots or opt into Rylai's and use it on an opponent that isn't too fast, and either way hope they don't kill Tibbers first within the 45 seconds her ult lasts. Overall, Annie normally just prefers to use it for the initial damage when she wants to burst someone down.

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which other ults cn one shot you at level one? I've seen others fully taken and havnt even died (channeled)

Simple. Veigar can one-shot an AP-stacking target with a level 1 ult. It's point-and-click. Rank 1 ult is 250 base + 100% Veigar AP + 80% target AP.

For damage-over-time ults, there are numerous champions that can kill you with just their ults at rank 1. AP Nunu can one-shot you (625 + 250% AP at rank 1) if you're foolish enough to stand in it for the full channel, same as MF. Katarina can 100-0 you (750 + 25% AP + 37.5% bonus AD) if you're foolish enough to stand in it for the full channel, same as MF. Annie's ult has a 980% max AP ratio, and that's not including base damage. Swain and Anivia ults can both do infinite damage as long as they have mana.

Ults are called "ultimate" abilities for a reason. They're typically extraordinarily powerful compared to other abilities.

MF's ult doesn't oneshot you from 100% HP and it has a warning laugh when she starts casting it.

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it wont one shot at early levels though. it depends on your ap and theirs

MF ult won't 100-0 you at early levels without additional AD.