Which build should I use

Yeah, I feel like Eliwood is good in theory, but his strange stats are difficult to work with.
This build should make him a good anit-mage/dragon.
Though his physical builk will still be very low for an armored unit.

Personally I would go with vengeful fighter with distant defence in the seal slot. And maybe a spd refine.
Might seem strange, but 39 speed is amazing for avoiding doubles.

I like your build, but I think arena (Which I'm assuming you're using this for) is more of a support world these days. I would probably say you're better off trying to feed your bonus unit kills, in which case take wary fighter and no qr. Possibly one of the spurs or drives in the seal slot and possibly c slot as well. Similarly, probably aether and a dual rally for scoring if you can. Overall though, the build seems really good for other modes. I think since LEliwood uses all 4 base stats, tanneboom will outclass berkut's lance by a bit (8 stats instead of 7 and speed is quite nice on him), but since tannenboom is seasonal it may be not fodderable for you.