I saw a youtube video of a battle and one of the pokemons moves was last resort but he/she didn't use all three other moves instead used one move mutiple times.I thought last resort only worked if you used all your moves.
Pika is right, but he said its not answering his question so, the video could be edited so only the good part is there or just a hack battle. Normally you have to use all other moves, so it could be a gimmick stradegy.
The only possible explanation is that the Pokemon only had two moves in its moveset. It is kinda a gimmicky strategy, but some people use it.