In order to fight Kela, you need to work up 25 Judgment Points in Rathuum. Stick to Nakki, as it's only the non-points rewards that change as the level rises. My preference is Ash with the Teleport augment and a dagger with Covert Lethality; failing that, a solid tank with a burst damage weapon, like a Rhino with an Arca Plasmor or Tigris variant, should also work fine.
Onto Merrow, the first obstacle is a platform with pressure pads, one for each member of your group. Stand on a pad facing out and there will be a rotating target; shoot the green section. Sounds easy? Standing on the pad deploys Rollers who can stagger you off the pads. There's a few ways to deal with this.
- Status protection, such as Rhino's Iron Skin, Nezha's Warding Halo, or Wyrm's Negate precept.
- Area protection, such as Frost's Snow Globe or Gara's Mass Vitrify, but not Limbo's Cataclysm, as you can't shoot out of it.
- Crowd control, such as Rhino's Stomp or Nova's Molecular Prime.
Once you're in the arena itself, Kela will drop in momentarily. She has two phases that she switches between. The first phase has her out on the field. She jumps around, fires rockets, and sics Rollers on you. This is a straightforward damage section.
Once you drop her health enough, Kela retreats and the orbital strikes begin with the second phase. Unless you're Limbo, get to the outer edges of the arena and keep moving until the strikes stop. You'll have about 15 seconds to get on the pressure pads that show up on the center platform and redo the target shooting from earlier. When the strikes start again, the targets temporarily disable. If you're Limbo, you can use Cataclysm in the center to provide a bubble of immunity to your team, but when it comes to shoot the targets, use it again to remove the bubble so your team can actually hit the targets. Once all targets are done, Kela jumps in and the first phase continues.
If someone goes down, use your newfound Operator power to revive them. Pop out into Operator form and crouch to go into the void, which also means you can revive without taking damage.
Last thing to note is that Grineer bosses use Alloy Armor as opposed to Ferrite Armor; Puncture damage is less effective (15% extra damage compared to 50%), but the ideal elemental attacks are Radiation (Heat + Electricity) for 75% extra damage and Cold for 25% instead of Blast and Corrosive.