"войнушка" пренебрежительно-игровой синоним (slighting synonym) слова "война" (war). I wouldn't say that was fun, as this was coaching the team in РЛ, the game of "war". Я думаю примерно так :)) P.S Не знаю что такое РЛ, поэтому не переводила.

"войнушка" it's not a real war, but simulation of war with simulating actions of real men and with imaginary effects.

No. It doesn’t mean “to play war”. It shows an attitude to the war as not a serious event, but likely as a game, as fun. And the author of this sentence wanted to stress that he didn’t deal with real war. For “ to play war” is direction translation «играть в войну». Children used to say to each other: “Let’s go to play «в войнушку» - Давай поиграем в ВОЙНУшку.» Olga called it as “slighting-fun synonym”. I am not a fun of RL. (( Good luck!

I would not say it was fun (amusing), because it was a traning as the part of the RL team, a 'toy/fun/stage war' so to speak. This can be a figurative language here. Without more context it's not clear what exactly they mean by this. I suggest you give us some more context.

РЛ (русс.) means “Russian Legion” (Русский легион). It’s a name of one of paintball team. I support the translation by Olga. I’d like to add that we said “войнушка” in childhood. Good luck!