Ya the shield is really Diana's key to being dangerous in the early game - because she can just walk straight up to virtually anyone early on and body block them at little personal risk - so if both mids suddenly rotate bottom to gank - Diana can just march right up to that enemy ADC and they need to back off or get E'd in for the kill: but her shield means she is at little personal risk doing so. Or if your ADC is about to die she can just go stand in the way and pretend she's Leona.
(Side note: even though they're supposed to be enemies, I find Leona/Diana to be a really strong support/mid or support/jungle combo - which is kinda weird)
You can do it with her build too - I'd say the biggest problem Akali has right now is that people still build her glass cannon and want to rely on the massive armor/mr she used to get in her smoke bomb - but she doesn't have that anymore and a lot of people never adjusted.I would say that Akali is better at picking off single enemies, because with 3 ult charges there is no fucking way you get away from her unless you are Zac of has TP. But if you jump on enemy priority target - you just see pink ward dropped and you silenced/stunned/murdered because you have no sustain to live enough.
Off course it's fixable in 5v5 environment when you can pick around Akali and give her some sustain
Try building Rylai's or Spirit Visage on her - Akali has high enough base damage and strong enough burst combos that if she can just stick to her target - she can kill them: even when she's built almost full tank. QSS is also not unreasonable for her if she gets counter-picked - you dash, bomb, they pink you, they stun you, you QSS, you dive their carry anyways
Like Evelynn (my girl), Akali has AP/AD flexibility - so virtually nothing that adds to either of those is bad for her - she has high ratios either way and she gains on-hit magic / spell vamp from doing so.