How did Cloud learn how to fight?

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?

What makes you think he's actually good at fighting?

A team of 3 renegades can't take out 2 grunt type Soldier guards easily at the beginning of the game (Shinra HQ etc).

He's got pure strength on his side, presumably from Jenova, his training (as a grunt), and his mako infusion. With enough strength to hold that sword, you realize he's pretty strong and so if he swung it at you, you wouldn't really stand a chance.

Unless you deliberately power level him, he's never really a better fighter than the others in his party, and you realise Tifa (only) seems to know some form of kickboxing so you'd expect his huge sword to be far more dangerous than her?