Zald, one of the mightiest in the Canon Zeus familia, He was strong enough that even he had the capacity to achieve a comparable combat strength to him. Despite being a level below him… Kinda nuts when you think about it…
Away, here we're going to have Zald also peak out in all his stats at every level minus level 7, where we will use his Canon stats.
Base of his basic abilities: Lvls 1-6 = 999 Each
Total: 5994 + 1800 = 7794
STR : 7794 + 999 = 8793
END : 7794 + 999 = 8793
DEX : 7794 + 902 = 8696
STR : 7794 + 673 = 8467
STR : 7794 + 515 = 8309
This puts all of his basic abilities firmly into the Lvl 8 tier, mid-high.
Next up… The other modifications we need to account for.
Afterglow +1 full level
I rank Supreme Attack +0.5 Level (This might be more, due to supreme, but better to low ball it.)
Deus Ambrosia
Al Questa
Niggle Gourmet
This gets him up to about a level 10 with everything combined. With the ability to have an outburst greater than this that would rapidly blow him away while using. So, basically equivalent to whatever the monster eaten is, or somewhere close enough to it.
Now, you may say, but the author said that he couldn't use the power of a level 10 with a level 7 vessel…
First, that is in direct conflict with other things used in the Canon series. Bell's slaying of the Revulsion Class Goliath, Ais' black wind during Astraea Record. Both are situations in which they get 3 levels higher.
Second, if 3 levels higher is too much for the soul to bear, then +2 is the absolute maximum, and in which case, the current gen has less than zero hope of beating the OEBD.
Third, the idea of +3 is too much is stupid on the surface, and underneath it. How else would Zald ever be able to do damage greater than Maxim's? Maxim would also have +2 through his various effects, and his base is a whole level higher. Thus invalidating the author's own words on the matter purely based on raw contradiction they, themselves wrote, to be fair, this sort of thing happens with basically everyone so do we take out of context statements as fact? Or do we take the series as fact?... Furthermore even with only +1 for Maxim, this would still be impossible for Zald, as in order to match him he'd need +2, and to exceed he would need more than +2, which is supposedly impossible based on the tweet.
Fourth, the only way for the comment to hold any water at all is if it means that Zald can achieve a maximum of what level 9s would be able to achieve for basic ability stat totals. Since the tweet does not reference it in this manner, we can rule that out as a possibility. Thus rendering it a moot point due to multiple events, from multiple people, plus the utter impossibility of anyone ever exceeding his ability. Yet… There are many instances of this…
So, no, Zald definitely can use the power of a level 10, and likely far beyond that for periods of time, depending on the situation, and at a price… And there we are once again in line with what's actually happened in the series.
Final Scaling:
Level 10
Level xx when consuming something to increase his status, temporarily.