Sometimes when I kill a lot of enemies at once some (like 2 or 3 at a time) become immortal.
What I mean by immortal is they don’t have health bars, aren’t affected by neither taunt or fear,but earthshatter still cracks the ground towards them and they also keep the poison appearance permanently. They also don’t get hit at all and (generally) don’t attack either.
I say generally because just yesterday I was doing a map with 80% pack size so there were a lot more enemies and it happened again, but in this case the lair druids (or whatever the ones that fire projectiles in the forest theme) kept attacking, and because the challenge map also had 200% enemy DoT and I don’t do too well in the health department, I lost my free revivals and worse, when I found the cartographer and attacked him with a bunch of enemies around, once again some enemies glitched too, but they also kept attacking so I died 3 more times and couldn’t separate them from the cartographer because they walked at the same speed so I had to buy a new map on the same level and waste overall around 1 mil in gold I was trying to save.
Maybe it has something to do with the lag I cause when killing many enemies at a time or maybe it’s something else.
Sometime your summoned fury can also become like this. Rather than immortal, I would say this monster became ghosts that haunts you 
. Although the chance to become like this is high only in pack size I think 
it all depends on your device. if you do a Search 
on Ghosts and/or Zombies, you will learn a lot about this.
one way to get past this is go to options and lower screen fx or anything else that would slow things down. check to see if any other apps are running. Wi-Fi off instead of on.
the device I am using now starts getting Ghosts & Zombies at about +125% to +130% Pack Size (max is +150%), so I usually use Diamond to get it around +100% to +120% when I am Farming. it took some time to test what Pack Size my device could handle before the annoying Zombies & Ghosts start to haunt me.
another thing is that you can lead them to one part of the map, then run away and leave them behind.
A bit of both really. Mainly I saw when things slow down.
A way to help as mentioned is lower FX if need be, disable Shadows especially as they are quite CPU Intensive. Low vs high used the same amount.
You can turn off power save mode if it impacts performance as they lower clock speeds to save power. Sometimes hotplug entire cores.
Wifi off which partly helps against background process and sparing processing cycles, check to see other apps or background processes running. More so on low RAM device to reduce chance of swapping which also takes some CPU. Also to have headroom for memory as Pack Size as additional effects use more dynamically than when idle. Game starts with 100MB but it can use 200MB+.
Testing what packsize you handle after that is a good general tip as well. Evem with Monster boost 100%+ packsize.
These tips can be applied to any device. iOS, android, emulator. For emulator/virtual machine, get the best performing one that you trust.
Basically what Golem say.
Rest tldr -
Some less general things could be reducing apps at startup on Android and iOS, reduce syncing, reduce some notifications, turn off GPS and or bluetooth if not used at that moment.
Also Force GPU rendering in developer options or set game driver as I believe this uses OpenGL ES 2.0. Game Driver may be able to increase openGL version or another improvement like GLTools work.
I’ve figured GLTools improved performance for me because I changed openGL to an unofficial 3.0 ES version or higher. As well as other tweaks to the rendering which may help. Tools like that do require root or jailbreak but some non root. It seems whenever I force GLTools opengl 3.1 when doing a higher adreno GPU, it runs better. Mileage may vary of course, as do other advance tweaks.
Reducing resolution may also help even though it’s a CPU bound game, and reducing texture size.
Do run the game on Internal Storage instead of SDCard, have enough storage available if possible. Since storage performance also mattered a little bit. Whether on emmc or UFS, it applies. This is partly why you want to reduce swapping as well since storage is gonna be slower than RAM. Also why I/O Scheduler sometimes makes a difference if able to change it. On Android or Linux. Since these root tweaks also work on Linux , with a sudo. Android based on Linux kernel somewhat.
Another thing I noticed is the game can run on Little cores which is fine if it’s capable but if it’s running slow, you can move it to the Big Core for an improvement. Especially A53 little core devices since that one is as slow as a Cortex A9 or A15. Which isn’t that bad for this game as they should be fine at normal speeds on a Galaxy S6 or other early big.LITTLE devices. Also the 32bit Galaxy S4 i9500 has the A15-A7 big.Little combo. Ideally you’d want the A15 used here.
Later Little cores should be signiticantly stronger on some SoCs. Snapdragon kyro 820 cores and above for e.g. As that performs like a Snapdragon 805/810 CPU wise. newer ARM Littles will be fine.
While the big cores are the true performance of a device. Little cores are more power saving. Other CPU tip is to increase clocks, disable hotplugging or not disabling cores. Whether on 32 or 64 bit.
Only reason to underclock or disable cores is if you have more than enough power. Long battery for longer sessions. GPU can be left idle as well since it doesn’t use that much. However, non root method with Power Save button is fine as well.
Last thing is heap size android build prop which makes a difference on lower ram or overall feel of android applications since they use dalvik/ART JVM. If not in Java/Kotlin, this tip irrelevant. I reduce from 512MB to 256-384 on 2GB mileage vary. This done from editing build.prop file either from root or adb push on a computer to Phone.
Can even do linux virtual memory tweaks but not necessary most of the time. For good default behaviour like how it Swaps.
Ok found a non root game tweaker, might be good. https://play.google.com/store/apps/details?id=inc.trilokia.gfxtool.free
Edit: woops didn’t see the date. A bit late posting this and some unnecessary unless really constrained.
I think it depends on your device bro, once your phone is lagging in a pack of monsters, expect a 2 or more zombies monsters, you can minimize that if you have an instakill hireling, because your phone will only start lagging if there is too much display of monsters in your screen, better build a hireling that will do the killing, if your in higher floors crushing flames set with crushing blow and immolate affix is a must and I add demonic set and electrocution to burst enemy heal maps, so fire and lightning is my favourite cuase it can go 1v1 with high ranking monsters, unlike frozen that you need to gather monsters to kill the big ones, if your in lower floors you can do poison. that’s all and happy gaming.