Sorry if I seemed condescending in any way. Your English is obviously very good I just find it interesting to see what things are called in other languages. And I tend to overexplain stuff because I've had people ask stuff like "what is the 'mystery'" before.

You could use Iruka over Guy, yes. I put guy in there because he's 3 stars versus Iruka's 2, which means he'll be a little more able to take hits compared to Iruka. Also, with Iruka, since both his and Sasuke's mystery skills can be used in round 1, and it would cost 40 chakra to use them both at a time when you only have 20, it can be annoying to have to make that decision on who to use. But really it's up to you, you can try both of them out and use whoever you like more. If you use Iruka, you should use these talents:

You should also work on getting the Kyushoku Deer summon (and all the other purple ones), and then use that instead of the small Katsuyu. When you get that, switch your chase to Lightning Sword.

In terms of ninjas you should go for next, try to get Kimimaro to 3 stars, or 4 stars (he gets breakthroughs then, and he can be a good early game pos 2 tank). Also try to get Sasuke to 4 stars so you can get his breakthroughs. In my opinion, there really isn't much worth pulling for in the regular treasures anymore, at least not any ninjas that will help you much in the late game. Instead, I'd suggest saving up coupons and spending to get one or two of the ninjas who are powerful in the meta right now (eg. Rinnesharingan Sasuke, Edo Minato, Habanero Kushina, Summer Kakashi, etc. These all require being 4 stars with breakthroughs other than Kakashi). Getting a high tier support ninja is also important. Edo Itachi and Summer Kurenai are good options.

Tho, I dunno if you have all the same ninjas we have right now or when other new ones will be released, I've heard different versions get ninjas at different rates than we do in the English version. Like, some places are behind us and some are ahead of us. So it's kind of hard for me to say who you should focus on long term when I don't know where the "meta" is at right now in the Polish version.

He can, and if you want he can be a great support ninja for Kakashi Summer, getting him 4 star is different as which I have done, spent money :)

Yeah I forgot about Halloween Kimimaro. If he's in your version you might go for him too. He is also good and you don't need to 4 star him or anything. He can be a pos 1 or support ninja so he'll be versatile as you get more ninjas over time.

Oh and if Shizuka is the current seasonal ninja in your version, I really recommend trying to get her to 4 stars (or at least 3) because she can be really good. Try to do as many of the missions for the seasonal points rewards as you can. Also, you'll see you can do the missions multiple times even after you've cleared them once, this will give you more points. Good luck!

This is the team I'd suggest with Shizuka once you get her to 3, or hopefuly 4, stars. She gets the swordsman buffs from Lightning main, and when you set up Gaara's barrier she'll get the Wind boost from that as well. Bee is there to give her an extra standard attack with his mystery and make her immune to interruption. He also benefits from the swordsman buff and gives himself and Gaara a shield and an increase to their ninjutsu and attack.

Shizuka's mystery and her scaling from getting hit are her main good points. Her mystery is unmissible damage, which is great, plus it supresses super armor, also great. If you get her to 4 stars you'll unlock the ability for her mystery to cause interruption if the opponent's chakra is 30 or less when you use it. Her chase attack also follows immediately after her mystery and adds an additional turn to the target's mystery cooldown time. All this together is really good on a free ninja. So do everything you can to get her outside of literally spending coupons or ingots.

With the combination of her being in 3v3 and being the seasonal ninja I'm sure you'll get her to 3 stars. If you want to get her to 4 you might have to grind a bit on the seasonal stuff. Like I said before, try to do all of the monthly missions to completion, not just to the point where the circled lights up.

The objectives might be different in your version but you can see here, for this mission, you only need to win 2 times in the arena for you to get the points for the boxes on the bottom of the monthly screen, but you can complete this task up to 15 times (so, 30 total arena wins) to get overall seasonal points. The more seasonal points you have the higher you will be in the overal server rankings, and the higher you are in the rankings the more Shizuka fragments you will get at the end of the month (at the end of May, if your seasonal cycle is on the same rotation as the English version).

Good luck!