It's not just lvl 5s, remember that jared attacks MC while we're distracted healing Laurie and the attack just bounces off, so a lvl 3 can't be harmed by a lvl 1.
I think we can extrapolate the following:
lvl 1 can't hurt lvl 3
lvl 2 can't hurt lvl 4
lvl 3 can only hurt lvl 5 if the lvl 3 is extraordinarily powerful for a lvl 3 (Like mr tree lieutenant)
I was just using Clark as an example that even without power reinforcing them, their bodies can't be penetrated by beings that could annihilate humans with effort.
We don't actually need to do guess work. The game and WW already put it out there. Level 1 and 2 can beat each other and can still be threatened by humans and their weaponry. Level 3 on the other hand can't be beaten by the previous 3 because of stat and power difference. The chance of a Level 3 losing to a Level 2 is so low that it might as well be zero. This is backed up by Ella and HERO.
Moving up, Level 3 is the crossing point where there's no cap on power that can be gained like with Level 2 and below. A level 3 can affect a Level 4 and 5, if their power is high enough and they're experienced enough. We have examples of this with MC vs Asura(Weakened but A-Rank monster = Level 4 to 5), Cole(Lvl 3) giving Nico(Lvl 5) a nose bleed with his power and finally Tree Boy being able to kill Hrungnir(A-Rank) with ease before fighting Ella(Lvl 5).
Level 4 would follow the same process as Level 3 but at a much higher rate since they gain power much faster and can close the gap quicker between a Level 5. We saw Ella "fight" two new Level 5s in Lexi and Clark while holding her own. This is because her power growth was already close to there's and because she was an Apostle Spawn. Level 5 need no explaining, they can kill all of the previous levels with little to no effort on their part with only exceptional Level 3s and 4s being a problem for them.
So to sum it up. Level 1 and Level 2 won't be able to beat Level 3 and above because they're capped on power and partly still underdeveloped. Level 3 and above depends on multiple factors like power trait, age in the particular level and how much experience they have. Example being Tree Boy would probably kill MC but MC would likely beat Cole's ass in a fight. Tree Boy has been in Level 3 for decades and has power and stats on par with Level 5s. Cole on the other hand is only 18 and likely only been Level 3 for 2 to 3 years and yet the MC already closed the gap, according to the MC's Apostle parent. With Level 5, Nico vs Ella and Nico vs Hex(S-Rank) was pretty even but Ella vs Malik and Malik vs Goliath wasn't even close.
Ultimately, Level 5 won't ever be beaten or killed by anything below Level 3. It's fine for some to say Level 2s can beat Level 3s because Ella left a chance there in her comment, even though it's astronomically low, that it could happen. Anyone who thinks a Level 1 can beat a Level 5 would have to somehow prove the game and WW wrong because there's nothing to back up that stance in game at all. If anything, the game flat out tells you a single Level 5 could end the world, if they wanted to.