Keep in mind that all of the fights can be significantly easier, or harder, based on RNG. Colossus may see fit to spam his shotgun blast, or legion may just run away forever. And yes, if for some reason you wanted to pick an optimal race, Slime People, and Diona are best. If you wanted to make each fight significantly easier just farm legion cores and bam, easy healing.
Kinetic Accelerator Mods
KA Triggerguard Mod - Allows species that normally cannot use the KA to use it (Vendor/Science)
KA Range Mod - Increases Range by one tile (Vendor/Science)
KA Damage Mod - Increases damage by 10 (Vendor/Science)
KA Cooldown Mod - Reduces cooldown by .32 seconds(may be wrong) (Vendor/Science)
KA AoE Damage Mod - Allows you to deal damage in a 3x3 (Vendor)
KA AoE Mining Mod - Allows you to break rocks in a 3x3 (Science)
KA Death Syphon Mod - Whenever you kill a creature, permanently increases your damage against that type of creature (Disk from Tendril)
AoE Hybrid Mod - Mod that does AoE damage, AND AoE mining (Disk from Tendril)
KA Rapid Repeater Mod - Greatly increases cooldown but reduces it to almost nothing whenever you hit an enemy (Disk from Tendril)
KA Resonator Blast Mod - Creates a resonator blast at target when hit, has to be detonated by hitting it again (Disk from Tendril)
Lifesteal Crystal Mod - Heals you for EITHER 2.5 Brute, Burn, or Oxy damage, priority in that order, whenever you hit enemies (Tendril)
Ash Drake (Fiery Signal)
Ash Drake is the most commonly spawning megafauna which drops the best loot of them all, however I believe they are the easiest to kill with the least amount of prep time. The loadout I run is; 3 cooldowns, 2 Burn Patches, a Medipen, and at least one Legion Core. The idea behind this is to force the drake into phase 2 as fast as possible, with the high DPS of 3 cooldowns. This is because phase 1 Drake has more consistent damage, as their firebreath is hardest to dodge here and is what you would end up spending most of your meds on. Phase two's fire breath is shot at an angle, and you will end up dodging it if you just run straight. If you manage to find the Lifesteal mod from a tendril, it makes the fight substantially easier, replace a cooldown with it. If you still struggle with Drake, substitute a cooldown with a range, and get more familiar with dodging him.
This is a pretty good showcase, as you can see that I am dodging horribly, yet still powering through the fight with proper medicine usage.
Blood Drunk Miner (Resonant Signal)
There is very little room to outplay the BDM, he will do a set amount of damage and there is little you can do to dodge it. However he has 900 health, and no armor piercing; Get a fully upgraded Mining Suit or Drake Armor and he tickles. You want 3 Cooldowns, a Medipen, a Brute Patch, and a Legion Core to prevent slowdown. Just run and gun.
Colossus (Angelic Signal)
Colossus requires a unique loadout of 4 Range Mods, otherwise you are just going to want the usual Medipens, Brute Patches, and as many Legion Cores as you can gather. Colossus will kill you in one of three ways; you fail a dodge, get too close, or run into a wall and make dodging very hard. Colossus is easiest to dodge if you fight him alone, and if you stay in one of the four cardinal directions from him where dodging requires you to only sidestep once. If you cut corners and position yourself diagonal from him, it will let him gain distance on you and make his death bolts more unpredictable and harder dodge. His most lethal ability is the 'shotgun' blast he will use, in which case you need to dodge immediately. If you tank 3+ death bolts at any given time, you can just outright die from getting stunned. The larger of an area you clear out to fight him, the easier, 70x70 should be enough. The death bolts that Colossus shoots can also aggro other mobs, INCLUDING other Megafauna, and they will aggro onto the nearest attack-able thing which is usually you.
Also, the circle that appears when you hit him means absolutely nothing, it is just a visual effect.
Legion (Echoing Signal)
Legion requires another unique loadout, of 1 Range, 1 Cooldown, and 1 AOE Damage. In terms of healing, you just want the usual Legion Cores, Medipens and Brute Patches. I would heavily recommend you acquire ash-proof clothing as well, as this fight is the longest of all Megafauna. Legion is a bit unique in the sense that, sometimes he will try and kite you, instead of you kiting him. Legion's attack pattern will involve him spinning and chasing you, and sometimes spawning little legion skulls and kiting back. You want to make a VERY long 1 wide tunnel in order to kite him through, in terms of gear, an ADV Plasma Cutter will help you heavily with this. Furthermore, after Legion has split, he will sometimes try to break through rock to FLANK you, he will do this especially often if there are nearby tunnels he can easily run though, to this end you want thermals to see when he is trying to do this. The AOE damage mod allows you to ignore the mechanic where he spawns little skulls, just run and gun, keep your distance.
The rest of the fight basically boils down to chasing the surviving legion skulls down, you've won at this point.
Hierophant (Zealous Signal)
I will add him if hes confirmed to be perma-added, but I doubt he will be.
Last Notes
If you have any requests or are still struggling with any of them, just @/DM me on discord.