I think I might top you.
For pokemon yellow, I had accidentally used my master ball and caught Articuno[1] (completely unneccessary). So when it came time to fight Mewtwo (there was no way I could go and buy more balls; if there was I didn't do it[2]) I went in with ONE regular pokeball[3].
I had to bite the bullet when I just trashed rare pokemon after rare pokemon because I couldn't waste my final pokeball. It was tough. For some strange reason I was not getting lucky, and every few steps was met with a random encounter. The majority of the times my escapes were futile AND my pokemon took some pretty recognizable damage given their level.
At last I find Mewtwo. Highest health was 50%. I knew this was going to be a battle of attrition: weaken him to his most vulnerable state and then, once the most optimal threshold could be reached (where anything beyond that would have been death; and there is no second chances), throw the pokeball and press "B" like a madman (you all know why...)
It was a vicious battle. The line up was a mix of high powered, high level pokemon, not so high decently powerful, low level weak and the line up would go tank (to bring him down as far as possible), then some middle guy to bring him down some more (but not too much) and finally a weak guy to bring him to his knees.
This plan, utterly failed. The tank did wonders; took out 3/4 of his hp. But everytime I made a switch, he would attack first and kill them. I was left with Articuno (ironic) and I tried to be strategic. Weak moves of quick attack and fly (respectively; probably only did it once) and I brought his hp down just barely into the red. Now was my chance. I scurried into my pack, enlarged that ball like it was a bull in heat smelling pheremones on steriods, and let that object fly. I was so caught up in the moment I forgot to press the "B" button like the incompetent pokemon player I was[4].
It jiggles....and jiggles...and finally....he breaks free...
God
Flipping
Damnit
I said "f*** it, you gonna die clown!!!" and I use blizzard on him...mutha'ucka's health goes down to one...
Guess who gets to start over 😐
What's the scariest game you played?