+Atk / -Spd Delthea

It depends on what you're going to do with your Delthea but in any case the one with +atk is better imo.
The +def even if it's a +4 is still at 17 defense which is useless and also it's reducing her magical bulk (with the -hp).
Of course you're more open to doubles which is pretty bad but you're a better glass cannon with a +atk than with a +def.
Giving her speed+3 as a seal to fix the -spd is a must I guess.

I was doing a study of blue infantry mages a few days ago. +def -res is, counterintuitively, a good IV, because she achieves 53 physical bulk with fury, which is often enough to withstand one counterattack from a DC lance other than Effie. +def -HP is....quite awkward, but it also works not badly if you use her with a blade tome and apply fortify def to her. The def buff gives her +4 damage and brings her physical bulk to 54 which, again, will enable her to survive many counterattacks unless you're throwing her against axes or lances with a bunch of merges.

At the moment, I find that Blárblade is Delthea's best option because otherwise Linde just has her outclassed, especially in spd, due to Aura's effect refine (unless you need the super-hone atk on Dark Aura). In general, due to that effect refine, Delthea is no longer able to keep up with Linde's speed (she was always two points behind to begin with), but Delthea is still a fast mage so she's good with Blárblade. (Also, who knows what refine she will receive?) With the spd bane, she drops into the upper end of Micaiah's territory (in fact, +spd Micaiah has the same spd), which means you're still looking to double slow units but it's getting into one-shot territory.

Which brings me to your other option: take the atk boon, keep her death blow, and go all-in with a one-shot bladetome build (basically, she becomes blue Lilina except she'll still double slow units as long as she survives a counterattack). With three hone/fortify buffs (one of which being atk), she has 39+13+6+16 = 74 atk, which will bust a lot of things. You can add atk+3 seal or even chill res if you want to go further. Otherwise, B tomebreaker would be an alternative (but it would probably be only for Micaiah and LA Lyn, because 74 will probably do the trick against Linde, Ishtar, Mae, Reinhardt, and opponent Delthea).