How To Clear Episode 4 Chapter 10 - 5 Zio First Encounter - Unbuffable & Decrease Hit Chance Debuff
The Most Useful Advantage the enemies have is buffs.
Damage suffered is decreased by 70%
Combat readiness increase
The functions of the buffs itself
Special Undispellable stacked buff
Considering the decreasing of damage suffered, you will focus on attacking Zio. Not bad, but don't forget about the special undispellable stacked buff. Everytime you attack him, combat readiness of one of his ally will increase. Giving him that buff, which will help Zio so much after reaching fourth level.
With this, you may choose to attack Zio's allies instead. Good choice. But, the decreasing damage is 70%. When you finally defeat them, it's easy for Zio to defeat your entire team. Especially cause of his third skill.
So, what is the safest way that leads to guaranteed victory?
Focus on both of them. Attack Zio when he didn't have so many buffs - thanks to that special buff - and his allies have two buffs each. Dispell their buffs or land unbuffable debuff or both. If it's succesful, focus on them. So Zio won't get that advantage from special buff. After defeating all of them, return to him. You just need to decrease his health until it reaches below 50% and it's over.
As for safety, no special requirement is needed. Just make sure that your heroes are sturdy enough, can receive enough healing ( don't rely on buffs too much ), and the injury won't defeat you before completing the stage.
Same with recommended heroes. Heroes with aoe attack seems better, but it isn't absolute. But if you still want a recommendation, then all heroes i used here are recommended. Especially Politis - thanks to her unbuffable and decrease hit chance debuff -.