Arenas aren't required for beating a boss but they're recommended.You can beat every boss WITHOUT an arena, some of my friends just disagree all the time and they don't accept my opinions (I already beat all the bosses in terraria without arena, except for Moon Lord which haven't came yet to mobile, I even know one who beat it without an arena with titanium armor only, he's in this forums, y'know, Vege)
well, now its an unpopular opinion! I think they are fine the way they are (picks).Vampire knives aren't so great, sure they're strong, but they're so dang unreliable to get in the first place that I don't really like them.
Also i'm not sure if this is unpopular but the early pre Eye tier is the most boring thing in the world, those ore pixkaxes really need buffs.