Hello! A friend of mine plans to run a homebrew campaign from levels 10 to 20 and I plan to bring the above mentioned multiclass to the table. However, I can't figure out how I should have the level split.

In terms of how I want the build to go, I hope to be a good damage sponge while being able to dish out some at least decent damage either at melee range or from afar via my echo.

Any and all advice is appreciated 👍😁

I would probably take one to 4 and go all in on the other one, depending on which one you want to be your 'main.'

Fighter 4/Barb 6 gets you most of the in-combat benefits of Echo Knight and gives you Spirit Shield from Ancestral Guardian and a bit more mobility. Continuing in Barb upgrades Spirit Shield as well as your Rage progression and you'll eventually get to Persistent Rage at level 19.

Barb 4/Fighter 6 gets you an extra ASI along with slightly more limited "Rage tanking" ability. Continuing in Fighter gets you additional ASIs, Indomitable, and a third attack. You also get Shadow Martyr, which is a lot like Spirit Shield. You get fewer uses but it can completely absorb some really big hits at key moments.

You could also do a more even split, something like Fighter 12/Barb 8 to get that third attack and extra ASIs but still upgrade Rage and get Spirit Shield. This feels a bit unfocused to me though and I'm not convinced it would be better than taking Fighter to 16, especially with Shadow Martyr and Spirit Shield overlapping in duties.

A few things to consider:

• You may have action economy issues. EK can be a real bonus action hog, especially if the DM likes to take it out. You will probably want to have the echo out before combats begin so you can Rage on the first turn. Also, take reaction abilities like Spirit Shield and Shadow Martyr with a grain of salt. A front-line "tank" has a lot of potential uses for their reaction, especially if you're interested in Polearm Master or Sentinel. As a DM for a PAM/Sentinel Paladin, it's quite easy for me to trigger a reaction early in the round and then just do whatever I want because the Paladin's reaction is spent.
• You will be an easy target for mental saving throws, especially the nasty WIS ones. You'll need 14 DEX for Medium Armor and will probably want to pump STR and CON as high as you can. This doesn't leave a lot for WIS. Indomitable can help with this, but when your modifier is +0 and the DC is 19, you will probably fail even with advantage.
• The Fighter and Barb capstones are both excellent. In a game going to 20, multiclassing at all has a real cost in that regard.
• More Rages per day has variable value depending on how your DM runs the campaign. If you often only have 2/3 encounters a day, extra Rages won't do much for you. In that cas eI would lean into Fighter.