Had a fair bit on my plate over the last few days, but I'll go over some relative basic behavioral patterns which shouldn't be too hard to implement.
Whether you use the more excessive Time.deltaTime loop (Which I do because I'm still slightly crap at programming), or WaitForSeconds() with a Mathf(Random.Range) for time delays, adding pauses to enemy attacks is critical. Right now what everything does is fire as soon as you're seen. Look at trends among Binding of Isaac, Enter the Gungeon, and Nuclear Throne. Unless you're going for Hotline Miami levels of time to kill, enemies generally exhibit a pause that can be brief or long, similar to how games that have enemies with hitscan weapons always miss the first shot, otherwise you'd be dead VERY fast. The yeti-robots in Nuclear Throne have a VERY long pause to them, but only most of the time. Their sporadic and aggressive behavior lend to some very interesting combat. When they eventually DO attack, the lunge in at VERY high speeds, but take a moment to accelerate (Except when they feel like ending my daily runs a little early e_e) and can be slowed down by taking damage from weapons with some knockback. Which is good because they do a lot of damage and have a fair degree of health.
To reference Enter the Gungeon, the Blue Shell-Kin fire in predictable (But still dangerous) patterns, after a brief delay of being in line of sight. And when they die, they have a chance to fire out shots in multiple directions. I fucking HATE the crows in the scrapyard in Nuclear Throne, but they've been well designed for their purpose. I have yet to determine their exact parameters for when they choose to fly to a new position, but I believe that it's have a relatively fixed period of time, they will fly within X distance of the player, regardless of line of sight. Sometimes this means they escape, sometimes it means they ambush.
And in Gungeon, it's about two frames long, but the guns of the bullet kin play an about-to-fire animation. Even though it's THE most basic enemy in the game, its attack has a tell. An important part to adding actual fights to Survived By is just going to be making some enemies tankier. Being able to 2-3 shot everything results in fights having absolutely no threat presence, particularly due to the fact that projectiles disappear with their host. So rushing down enemies is ALWAYS the way to go, because if you can kill them before the shots reach you, then you were never in danger in the first place. Things just happen too fast right now. It's hard to build up any RPG elements and actually use them if offense is the only thing that really matters, and you have such a low TTK. Like I said with Hotline Miami. There's zero reason to EVER put in RPG elements because there's really nothing to expand upon besides the singular-trait masks. (Tony and Jake OP) Which ALSO comes down to speeding up the TTK to limit your exposure to enemies. Tony is killing punches and instant finishers, and Jake is killing throws, so even if you don't have ammo, having any weapon in your arms is a "safe" long-range kill.
I REALLY TRIED to play Infiltrator before the Open Beta "wipe" but constantly I found myself in a state of frustration because she (All of my characters are female, no one can tell me otherwise) couldn't do even remotely enough damage compared to Harbinger, whom I still main because... Well, she's OP. Why have to get in close to use an ability with limited range on top of its short flight path when I could just stay at a safe distance and pound enemies with an auto-lock shot? This is again due to the enemies we fight. The Harbinger playstyle is simply the most effective given the behavior of enemies and projectiles.
Also enemies don't interact with each other NEARLY enough. I have yet to find proof of this theory, but it feels strongly like the basic bandits in Nuclear Throne fire more frequently the more of them are nearby, like a sort-of confidence aspect. (No proof though, it just *feels* like it) and there are enemies in Gungeon which buff each other with damage and/or damage resistance. And enemy targeting priority would be a good plan. Say, some enemies will mainly target the player with the least health or something. That's about all I have to say for now, despite the fact that I could give some more concrete examples of actual designs,but I may do that at a different time.