In the main series games, Jolteon is a glass cannon, it's fast, and can hit it's opponents really hard, but if it doesn't OHKO, it can take some heavy damage, but Speed has a healing factor, so he can take hits, and with Turbo boost, he can take down Ground types as well, so I'd say he's more of a Steel cannon, defeatable, but it's difficult.
I don't think that Speed is defeatable, even tho his enemy manage to kill him, he would simply revive and finish the job, now that I'm thinking, the only way of defeating Speed would be sealing him or something like that... Speed is OP!
His healing factor wouldn't heal fast enough to stay in lava, and in space he would die asphyxiated or at some point hit a planet, asteroid or even a star. meaning that he would repawn back.