fodder or keep +spd/-def legault

Keep him, he's really good and if you need a unit for AA he's up for grabs. and wait for a bad IV of him.
but if you don't care enough for him then fodder him off.

Wait. He can be demoted and your IVs are ok

I've really enjoyed using him for the TT but there's only a few days left on that now, so...

Your idea for attack tactic sounds very weird to me. You want to put it on Lute, an offensive unit, to buff Axezura, a buffing unit? :D

If you want a dagger unit, Legault is one of the best options around.
Given that he's 4* now, you could just use the next Legault as fodder.
Of course if you don't plan on using him, it's not a big loss if you fodder him.

Legault hast Atk Tactic at 4* now so I recommend waiting it out. I also won't advise giving expensive skills on rare units with low bst unless you don't care about saving resources and arena scoring.