This always struck me as a major plot hole in the original game. The facts were established:
Cloud has Jenova cells in him and mako conditioning. This gives him strength and regenerative abilities
Cloud stole Zack's identity
Cloud was a grunt, not even a low level SOLDIER
Cloud was incapacitated and only had a few months to recover before the events of the game
None of these seem to be strong arguments towards Cloud learning any actual fighting ability. One could argue that he was influenced by Zack or Zack taught him a few things, but this does not substitute for the type of training say Zack received in SOLDIER. Furthermore, Cloud fought with convincing ability. Though none of the members in AVALANCHE probably knew what an actual SOLDIER's capabilities were, Cloud seems to have no issues fighting, not even against low level SOLDIERS when they raid Shinra HQ.
Is there any canon that establishes how Cloud learned to fight?