1. They are fast, not greaver crazy but they have no problem putting you in range of their bullets. Their bullets are also fast, they deal status reliably and capitalize on server lag much better than any other monster.
2. They are dodgy, when it comes to your average ranged monster you pull out whatever gun you have and shoot it apart, devilites will dodge most of your bullets and they capitalize well on your reduced speed while reloading.
3. Regenerative health. When a devilite transforms it gets all it's HP back. In one floor devilites generally go through 3 transformations with 2 being hostile. (and the other too tanky to focus on)
Devilte>overtimer>Boss>devilite (if a boss is nearby this chain can continue)
Devilite>Boss>Devilite (if you kill a boss a new one is transformed)
Devilite>Yesman>overtimer (he can also become a pit boss)
Boss>Devilite
I can excuse lichens because they have unreliable ranged attacks and focus mainly on melee attacks.
4. Lack of piercing weapons
We don't have many
2 swords
2 easily dodged guns and one with low DPS
3 bombs (2 are shards)
Only the swords are reliable when it comes to beating devilites.
Devilites are some of my favorite enemies to fight. This might be in part that I actually wear defensive armor so their hits won't throw me into a status panic.
But when it comes to fiends, and devilites especially, you need to think why they are hitting you. Why are devilites hitting you? Because of A) Server connection, B) proximity, C) Speed. Then you need to compare that to their weaknesses. Devilites are A) Weak to piercing, B) flinch with just about any hit, C) Can and will dodge into bullets.
When I know I'll be up against a few levels with fiends in it (Fiend stratum, Arena, or a dark city, with not many danger rooms at radiant depths) I typically only actually have one piercing weapon with me, and that weapon is a dark briar barrage. That bomb is too good against both beasts and fiends. I will also bring along my warmaster rocket hammer, magma driver, and DVS.
See, what you need to take down fiends is to interrupt them. Fast weapons will trump slow ones. With guns, you need to aim either at them or where you think they'll dodge depending on bullet speed. Any sort of driver is great for this because it will ricochet into the devilites.
Fiends are some of my favorite enemies to fight. They pose a challenge if you're not focused, and will be the end of you if you try to walk in without some sort of defense against them.