Angra Mainyu

You do know Angra Mainyu is a joke servant, right?
He may be useable, and in that sense, if you just want to make the game more more challenging. But why fight the meta?

His 1st skill is very niche and only good against crit-spamming enemies.
His 2nd skill gives a bit utility but not anywhere to justify using him when other F2P servants can do what he does, but better.
3rd skill is actually good, if only it doesn't kill him on the 5th turn (Interlude completely mitigates this, but it's still a long ways to go).
NP is utterly useless in the long run when you start facing enemies that can potentially one-hit him with just a basic attack, and by then his NP won't work.

But hey, he's Avenger. He doesn't suffer from normal class affinities. Problem is, there are only a few Rulers in the game for him to take advantage on.

Avenger need LOTS of materials. No kidding, his demand can be compared with Jalter, and other SSR servants (even his Ascension need a lot)

He is decent as star generator, and Avenger normally wont adsorb star so equipped with 2030 he "probably" can send star to other team (well, I dont want to try it because I have two Angra Mainyu but really dont want to invest him due to large amount of materials). This effectiveness can be combined with his 3rd skill, but problem is you have to wait ~1.7 years later until this skill strengthening to avoid self-suicide.

And to make his NP works, paired with Kiritsugu is not a bad idea. (also Iris to make his survived, so overall a MEME couple team for Fate Zero). And make his 2nd skill worked in such durable fight.
Beast attacking when bond CE obtained (but really hard)
So overall you will find it is really hard to build a team for him and make him worked great. I wont deny a stronk team made of him and other 3-10 Kiritsugu+Iris, or grail him as lv100, but is it really necessary?

Besides, I cannot think anything he is very solid for. Maybe 2nd skill and equipped with Taunt CE?

And not so many bonus besides of Fate Zero, or maybe specific event for "extra" class if you dont have Ruler/Other Avenger.

If you really like him, I find a solo match for you
https://www.youtube.com/watch?v=-pg8RAbez90

He’s really not worth it, I invested in him knowing I’ll probably never use him because I wanted to level him up for the heck of it. But seriously, he requires a lot of rare mats and most people don’t use him for good reason.