Please know I respect your opinion but our perspectives differ on the matter, I've answered this a few times so I'll be brief this time.
1. I don't level to Epic unless an event forces it..so my Grand Uni is lvl 10, my Opal octopus is 8 or 9, etc I haven't battled with a lvl 4 so can't comment on it, but lower level animals do fine if they're the right element.
2. We differ on the importance of rarity as well, as most of us have seen in result posts, a common (basic single element) animal can take down an UR easier than an UR or SR at times.. Again in this game rarity doesn't matter in my opinion.
3. Fatigue. A few say they get fatigue a few don't. I have all my regular arena battles completed, therefore I'm not using the same animals to do battles AND tournaments. Perhaps this would cause fatigue. One long time player said their animal seems to get fatigued at around 50 battles doing both arenas. As you've seen ever tournament, I use the same animals, and many times 3-4 times in a row, with no issues. I sometimes switch to test other animals but I don't hesitate to use one I know is good back to back. My suggestion to those still doing battleground is to NOT do it during a tournament, it's permanent so it'll be right where you left off
4. I don't use the 30 gem nuke, no need. I use skip each round (4 gems) to complete it and share my results with you guys. But as I said in my lengthy post explaining all this, a 30 rd tourney costs 120 gems, say you fail 5x add another 20, now that's 140 (192 in 48 rds) if ppl cap their limit at 200 they're still ahead, you get very far then have 9 more days to complete less than half! So my thinking is, these animals sell for 1000-1500 gems in the shop, I'm getting it for less than 200! And I'm free to enjoy my weekend because I hate cking in every hour..stress free!
Plus I've found if you're paying to skip through, you do better. Yes I've had fails, they're just rare instead of constant. I put them in my results when they occur.
5. Some of my animals do better than others, some are finicky and rarely go critical while others rarely fail. I've seen this before in battle type games. Now this is speculation because I'm not a dev obviously lol but many devs put in a hidden stat that comes up with the RNG when animal is bred, or activated in to game ie purchases. Like a % of critical hit rate on that animal, which would make some great and some run scared from a fight! lol Example would be my Grand Uni being awesome and yours doesn't want to play, so if I were you I'd try Opal Octopus in that case. Like I said speculation from my testing and not a "fact" divulged by devs because they aren't going to say. Just like we know it's easier to breed an animal needed for SB before and after that SB but extremely hard to breed when SB asks us to. They denied changing % of the needed animal during events but proof is in every SB
My suggestion is go by the element ppl post to win a battle, you pick your best (meaning hits critical like a beast) animal of that element, paying attn to your secondary element that can slam you into neutral and you lose if your second isn't strong or neutral against enemy.
Sorry this post is going beyond "brief" so I'll close it with a summation: you don't have to use epics to win.. 8 and above are good they work for me just fine. Pay attn to elements taking secondary element into consideration, sometimes it won't matter you can have neutral secondary just try not to have a weak one or hope you get very lucky! Skip through as many as you're comfortable with IF you choose, rarity doesn't matter, one of your great battle babies are waiting in the wings to fight and gets passed up due to rarity, test them! You'll find you'll come up with a core battlesquad and use others as needed to test or whatever-everyone plays differently there's no right or wrong way..I'm just explaining MY way lol
My apologies for going long, Ty for reading and most of all GOOD LUCK!
Add: I'm lvl 142, I haven't noticed it being more difficult according to lvl, low levels have common, rare, SR, and UR same as higher level people..I did well in tournaments at a lower level and not having issues now. As I stated above I'm convinced the RNG (random number generator) throws out the % rate that animal will get critical as well as the other categories. Plus I have horrible lag issues which can and does affect everything I do in game, if it lets me stay on to play haha!