If the missfire is evident on tick over, then all you have to do is unplug the coil packs one by one till you find the demon.
You would have to have done this to establish which coil pack to move in the first place to check if the missfire followed the coil pack.
Then it is a simple case of eliminating the coil pack, plug, compression issue, injector, in that order.
However code po300 is a random missfire and could be related to numerous other things, but the above is a simple starting point.
Alex.