Thankfully, most of the episode isn't about the monster for a change. During the investigation, Zed has a seizure out of the blue and finds herself admitted for medical care. A cat scan reveals the surprise news that Zed has a brain tumor, which may even be the source of her psychic powers. Zed now has to decide whether she'll have the tumor removed, saving her life but cancelling her powers, or leave the tumor in play and risk an early death.
Constantine actually blames himself for Zed's plight. Magic has a cost after all, and by forcing Zed to constantly use her psychic powers he fears he's exacerbated her condition. Losing another friend isn't something he was prepared for, so he spends most of the episode avoiding what he's "done" to her. It's a great character moment, but the best part comes later: Constantine confronts Manny and demands he heal her. Manny insists he cannot, as usual, but Constantine won't accept no for an answer. So he breaks out the big guns - a powerful binding spell that traps Manny in his host body. This ingenious little twist comes out of nowhere, showing how dark Constantine truly can be and how deeply he's come to care for Zed.
With Manny bound, Constantine's intentions are straightforward - force Manny to spend more than two minutes at once on Earth and actually experience a range of human emotions. In practice, however? Manny is now the comic relief, vomiting at murder scenes and awkwardly stumbling into sexual encounters with his host's girlfriend. (I'm pretty sure that's borderline rape for both parties in this context, even with a throwaway line that the host enjoyed it too.) Constantine crafted this dark moment with gut-wrenching implications - John knows what it's like when angels are cut off from divinity - and it's used solely to humiliate and traumatize Manny for humor. Oh, plus it cuts Manny off from his angel power, which is the only thing that can stop The Black Diamond.
Remember, Constantine's Manny is not Hellblazer's Gabriel. Manny has expressed genuine emotions to John before, and sympathizes with his cause. It's only the divine mandate that kept Manny from directly interfering, not a lack of will. In that context, Constantine's actions are especially cruel - which would be an interesting direction if we weren't supposed to laugh at Manny's plight. Speaking personally however, it just left me cringing.
Anyway. Eventually the Black Diamond is defeated thanks to a yawn-worthy Deus Ex Machina sequence, while Zed risks an aneurysm and decides not have the tumor removed. We're left with Constantine explaining his standoffish behavior to Zed, then Manny wrecks the mood by suggesting Zed's powers aren't evil - they're part of God's plan. Without, you know, actually confirming the tumor has anything to do with her powers, or mentioning how long she has to live.
"Angels and Ministers of Grace" is trying to have a feel-good ending where Zed's starry-eyed belief can be justified for once. But sorry, Constantine, this isn't Touched by an Angel - mostly because if this all was God's plan, I'm deeply concerned for the rest of the angels.
Bottom Line: "Angels and Ministers of Grace" features the best character developments for Constantine and Zed we've seen yet. Sadly, it also squanders them for a ridiculous monster design and Manny's unexpected comic relief. As great as the better moments of the episode are, it's not enough to salvage a confusing plot and out-of-place religious tone into something truly satisfying.
Recommendation: Constantine purists will love Matt Ryan and Angélica Celaya's performances. Otherwise, unless you're watching to laugh at the B-Movie monster, stop the episode after Constantine punishes Manny. You've already seen the best parts by then.