Help zelgius

Zelgius really works with any IVs. The -Atk sucks but the +Spd makes it better preventing him from getting doubled in a lot of situations. He's still very usable being able to reach 55 atk with fierce stance so you don't have to worry.

Black Luna means Zelgius hits hard anyway. Even more so if you have steady breath+Wrath or Brash/Vengeful Fighter. But even without those he should be okay running with armor buffs.
Of course, you can also give him ATK+3 SS to mitigate his bane.

I think even if he had base attack stat of 0, he will do more damage than a lot of heroes thanks to his special. He hits hard even with -3 attack due to his already high attack stat and 16 mt. Not to mention his A skill... I would not worry about it.

You got -atk which is not very good but that +spd more than makes up for it. He still has 55 attack thanks to Fierce Stance. You can also try Fury, his atk bane would be patched up and he would have 39 speed, which avoids a lot of doubles, even from a neutral spd Mulagir Brave Lyn(you still wouldn’t be able to counterattack, but you avoid the double attack). He still keeps his amazing bulk intact.

Fury/Vantage is a possibility, although his base kit is still pretty good(Fierce Stance/Warp Powder). Steady Breath and Vengeful Fighter are pretty nice options as well. He can be a great sword for your Arena defense team, because with Quickened Pulse seal, if he has Vantage and survives one encounter it pretty much means they will have to face DC/Vantage/Black Luna, and there are not a lot of units that can survive that.

So no, he’s not useless by any means, he’s still pretty good, and you have a speed boon which is, if not the best, one of his best boons.