Well his armor is 2nd or 3rd tier level at best so going against first tier enemies and power isn’t saving the armor. He’d need top tier armor for it not to break

Bell armor was created by Welf and he has a contract with him. Asterius is estimated as lvl 7 - why would lvl 3 adventurer invest massive amount of money to buy armor cappable of blocking that kind of attack. Probability of encountering this kind of monster is near 0. Even when armor protect you if attack is strong enough you would still feel it and could even die.

On side note author often use armor as something that just break during fights.

Regarding Hestia knife - it is best weapon for adventurer that is still growing. Bell was still lvl 1 when it was created and noone expected him to progress that fast. During fight with Asterius it was lvl 3 knife. It was obvious that it would have hard time damaging lvl 7 monster. With time and Bell growth it would become one of best weapons in Orario. In Ln 12 we see that Argo Vesta improve it range a bit. I think it resonable to suspect that with Bell getting stronger that technique would have greater effect.

Ps

Hestia knife was created by god of smithing there is no way that someone like Welf could improve it right now

Hestia knife grows with Bell so at lvl 3 its not going to deal such a big dmg to a high level enemy. Also they are already in debt because of the knife. Welf is a talented blacksmith but still lacks knowledge. Tsubaki could eventually help out make something, or Hephaistos because Welf got the Crozzo blood. Even though if i remember correctly it was stated they made exepcional magic items dont know about other items.

First, regarding the armor. The armor was made by Welf and it is light/medium armor which suits Bells combat style and Bell prefers this type of armor.

Being light/medium armor, the amount of damage it can is less than what heavy armor can take. Plus we are talking about attacks coming from a opponent 4 levels higher than Bell.

Even if there was a light armor that could handle that, it would be seemingly unnecessary as Bell is a level 3 and the worst any person might expect him to go up against is a level 4 monster.

Now for the Hestia Knife. The Hestia knife is a powerful weapon that grows with its user so its strength reflects Bell's level. Again, we are talking about a level 3 adventurer facing off against a level 7 monster.

The weapon is more than fitting for Bell as he never has to replace it. But given who Bell was facing Ottar threw him his weapon. That is a weapon of a level 7 adventurer so you would expect it to be able to damage Asterius without being damaged in return.

Basically, what I am saying is that there is nothing wrong with what Bell has because no one would expect him to have to take on such a powerful opponent.

Just wanted to add a few things to what @Chronicler75 said. @ProVizard your reference to Ais's desperate is kinda pointless cause Desperate is made from Orichalcum, the strongest mineral in Danmachi which is stronger than Adamantite. It is a very rare metal and extremely expensive too. For a level 3 Adventurer it is too big of an investment especially for something like armour.