How to suck less as a melee: Know your foe.
Find out as much about these four stats it has, as you can, Vit, Agi, Def, Eva.
It's Vit is important because if you can't get your Str higher than that, you will suffer a penalty on your weapon damage, conversely, the higher your Str gets compared to its Vit, the more base damage you add on.
It's Def is a bit less important if you're relying on enspells/poison/etc, because where Str vs Vit gives the lowest and highest numbers you will hit for, Att vs Def tells you where you will wind up between them generally.
Agi effects your Critical hit rates, this matters for Evisceration quite a bit, you want just enough Accuracy to land your hits, and as much Dex as you can get since it's not only a WS modifier (which adds the given % of that stat to your weapons damage, with a bit of level base conversion), but it Dex vs Agi determines your crit rates.
Eva is related to Agi, and is simplest to get, if it's too evasive, you ain't hittin shit, and everything I just said flies right out the window.
If you want more technical breakdowns, just ask, but I must say I'm curious what Mandau would enhance with a higher level solo enough to make it worth it, but I'm not big on questioning relic choices, been rooting Izzy on since I found out he was upgrading.