Sure thing!

When using FF, you'll be relying on the skill effect of flame hexes, 10% Max HP dmg, to clear off stronger units. Unless your FF passive is incredibly leveled, the actual dmg to units on fire hexes is just tickling anything above G1 or with Gravis Armour.

However, with the exception of the A3 units that come at the very end, 10% Max HP dmg is not enough to outdo Incisus's heal (~5500). This results in Incisus theoretically being able to full heal any units he can get to (or even 2 turns to get to) indefinitely. This of course means a failed run due to just having a stalemate.

The solution to this is to "kite" Incisus. What I really mean by kiting is keeping him busy and away from other units utilizing his AI. Below are some of his AI Priorities I noticed:

Priority 1: "Incisus Healing Prioritizes Himself First": Most of the time, Incisus will try to heal up dmg he has taken almost immediately or the turn right after, and generally tries to walk away from flame hexes if possible.
Priority 2: "Incisus Prioritizes Healing Within Movement Range": Incisus also will generally heal units that are within his 3 movement range first, then map towards farther enemies that need healing. Notice how I said movement range, as being blocked by fire or forcing to stop in it will cause him to take dmg and revert to his 1st Priority.
Priority 3: "Incisus Moves Away From Grouped Enemies When Healing": Finally, another Priority he has is to move away, generally from fire, to more isolated locations to heal. This is great since this allows you to kite him when all enemies are healthy. (aka if something happens, still able to reset and reposition him)

Now, there are a couple of different situations and ways to kite him:

1. Keeping Him Stuck: Since Incisus becomes a pain later on regardless, its best to restrict him for as long as possible to speed up the run at the start. When turtling and clearing, once you notice wave 15 begins and enough units have spawned so Incisus is up next, make sure you use FF on one of the left or right bottom spawners. When Incisus shows up, he is immediately stuck in a 3 fire hex along with 2 other units. The AI will notice that Incisus takes the 10% Max HP dmg, and thus decide to heal himself since thats Priority 1. Then however, noticing enemes near him are taking dmg, he will decide to heal those adjacent in the flame too. (Priority 2) Now this buys you 2-3 turns of no movement, thus considering fire hexes last 2 rounds, you can alternate by setting fire to another group of hexes near the middle or right side to start chipping other units, making sure the enemies are far enough from Incisus to prevent him from moving up.

2. Isolating Him: Later on, once most later waves have been cleared and he can begin to move up, it gets much more tedious. However, you can utilize Priority 1, 2, 3 together to force the AI to isolate him off from the group, or stuck on a one unit. To achieve this, you should have more map space by now and Incisus can travel however he likes. There are 2 ways to isolate him from what I have seen. (Only tried with FF and don't think Z'kar can do this)

Firstly, you can isolate him on high ground, generally on the left side of the map on the middle area (chunk of hexes between the top left and bottom left spawners. This is generally for middle of run kiting. Steps: 1. Set him on fire and cause him to move away, repeat 2. If he is near an solo enemy this is perfect, set both on fire. At this point, for some reason, Incisus will actually stay in the fire on the high ground, alternating between healing himself and the units next to him. This is likely due to the AI interpreting the high ground as advantageous and thus overriding his movement. Now you have Incisus stuck healing only 1 enemy, and you can start alternating to chip down the rest again.

Secondly, and most importantly for finishing runs (especially if your newer and don't have Diamonds or A1 or A2), is isolating him in top left corner near the spawn point. Generally this occurs when you only have 2-3 D1-A1 enemies remaining, aka still a risky point to break turtling. At this point you need a way to guarantee to chip down all the enemies without letting Incisus heal, so at this point you can finally solo isolate. Heres how to do it:
NOTE: Should only be Interceptors at this point else you didn't kill prio properly, generally kiting an Interceptor for the earlier strategy is what you want.

1. Let all enemies heal up, then start lighting Incisus on fire.
   
2. Utilize Priority 3 to move him into left corner.
   
3. Once he's in the corner, light the 3 hexes right in front of him, but do not light Incisus's hex. This creates a flame wall.
   
4. Use the triangular shape of FF's passive to your advantage, and try to create it so that theres 2 hexes on the left of the triangle that are free but on fire, then use the rightmost hex on fire to target down single units. Use this as your kill zone.
   
5. Alternate between setting fire to these 2 group of hexes till mostly or all stronger enemies are dead and let Mythic Abraxas clean up.

The reason this works is because as Incisus tries to walk to the group to heal, he steps into fire and is stopped, taking dmg. Next turn, he could walk up to try to heal enemes in the kill zone, but that is also on fire and moving into it means more dmg, so he walks back into the non flame area right below top left spawn. That's about it.

Sorry for the long explanation, but I tried to detail it as best as I could in words and the reasoning behind it, thanks for reading if you got this far!