HIIT sprints. (Tabata or similar protocol)
That should be all if your goal is improved stamina.
e.g. Tabata;
Sprint, full out, for 20 seconds,
then jog/walk for 10 seconds.
Sprint 20 seconds,
jog/walk for 10.
repeat this for 8 cycles.
And when I say Full out, you literally want to push it as hard as you can.
And no cheating on the time.
of course the time-signatures can be altered to suit your needs, so if you would usually need to sprint for 30 seconds during speedball, then use that as your interval and extend the "rest" intervals by a little as well (maybe up them to 15 seconds).
But once you've decided on time-signatures for the work-out, stick to them even if it means running till you feel faint.
This workout should take you something like 10 minutes, but is among the most effective exercises you can do. (also it is scientifically proven to be at least as effective as tediously running for 1-2 hours at lower intensity. (jogging etc.))
I'll see if I can't dig out those articles comparing the two.