You have to know you're opponent and how they fight. Race and fighting style are critical here and can give you an idea on how the individual fights.
Ottal didn't "easily" counter Bell's attack, he had to resort to Hildis Vini (An attack required to defeat Zald) and is Ottal's most powerful attack when combined with his immense strength but was parried with Bell's fully charged Argonaut and resulted in a draw between the two. He admitted that Bell's attack would be superior if not for the level gap, but regardless it was still a draw.
Argonaut-powered attacks are not to be underestimated, if what you say is true then Ottal wouldn't need to resort to Hildis Vini to counter, but he did. Another thing to note is that Bell's second charged attack was only 1 minute and not 5, otherwise Ottal would be a LOT more injured or even dead by taking on a second full powered Argonaut attack from him. IIRC, Ottal noted how dangerous it was if he had gotten hit a second time if it was a fully charged attack and KNEW it could kill him.
Back to Aiz, she is only one level above Bell, but Bell on the other hand is dangerously close to surpassing her. In fact, in terms of stats, he already has, and he wouldn't even need to level up to face her in a more or less equal match, all Bell needs to do is max out as a level 5 (and would be interesting thanks to Lyu's new buff skill) and fight her. Aiz' only ace up her sleeve is Ariel but Argonaut is far more powerful in terms of one-shotting potential and raw power.
EDIT: Below is part of an estimate made by a redditor.
Currently Bell is;
1888 superior in Strength
2071 superior in Endurance
656 superior in Dexterity
1518 superior in Agility
440 superior in Magic
With the level up bonus (estimated at 999 per stat) taken into account Bell is probably significantly stronger, more durable and faster than Base Aiz, if she isn't using Magic he only loses out in Dexterity, and not by much.