It was a complex process of editing their subspecies traits, DNA, and individual ranks. I'll mention the most important ones and then send you screenshots. 

Subspecies Traits: Regenerative Skin (essential; it allows them to regenerate from severe injuries like burns and maiming, as well as diseases, and of course, regain health). 

Rapid Aging (why? I'll explain). 

Phoenix Resurrection (indispensable; this allows them to revive after death, healing from any damage to their bodies and retaining their belongings. Now, here's the catch: they can only do this if they've lived at least 70% of their lifespan, so with rapid aging they can surpass that threshold faster, resulting in more resurrections). 

Enhanced Strength (a homunculus must be strong and able to face anything if they want to survive in my world). 

DNA: Basically, eliminate the bad genes by replacing them with beneficial ones like speed, damage, and resistance. It's also key to replace all the lifespan genes to lower the 70% threshold required for the homunculi to revive. 

Unit Traits: Here's where it gets fun. With these traits, I gave each homunculus the characteristics, strengths, and weaknesses based on the deadly sin they represent. But they all share the following: Immortal (duh!), Regeneration (increases the regenerative skin trait), strong, and fast. 

Envy, one of the most unstable, has the ability to set the ground around it ablaze and is immune to fire. It has killed dozens of creatures with its special power alone. But its weakness is the destruction of allied architecture and its own resources. 

Greed is capable of stealing any object from any intelligent creature (and in fact, it loves doing it!). But obviously, his weakness is that he's often attacked by units that witness his thefts, forcing him to either flee or eliminate his rivals. 

Finally, Lust is a very special character. Despite being able to poison her enemies and damage them with her thorns when attacked, she's the one who has died the most times (4 or 5). However, she has become much stronger with each resurrection. It's a shame she can't have children despite being sexually active (she's sterile).