edit: my jael only can do Brutal stage 6 (NOT 7)

Kael 50 should be able to farm 12-3 on brutal, even without all masteries. Probably you should check your artifacts (i think, i used lifesteal from "new player rewards" and cruel).

I personally did Kael 60 first. And after that Ninja.

Personally, I would do Ninja first, than Tomb Lord...if you are looking for a campaign farmer...Bellower might be a good option as well. 

Tomb Lord is underrated but have a look on all the debuffs he is throwing out 

Bellower can be a faster camapign farmer than Kael - if you have the right artifacts on him. Bellower has a lower attack stat than Kael, lower damage multipliers and no damage enhancement from books on his strongest hit (A2). In early gear, that a new player has at the start, Bellower is a terrible campaign farmer.

It's the same with Saurus and some other of the "fastest" campaign farmers. Youtubers with their whale accounts say champs like Bellowers or Saurus can farm 12-3 brutal in less than 10 seconds - their Bellower and Saurus with 300% critdamage can do so. But it's nothing than a noobytrap, with the artifacts you have right at the start, it's more a minute long campaign farming with these champs. The difference of one or two shots per wave is gigantic, because the enemies get their turn with their animations if they don't die from the first hit.

I would just do kael.

He is good enough to campaign farm, nuke in arena, poisoner for clan boss, faction wars, you can run him in all the dungeons at least through stages 13 to start getting five and six star gear.

He is rare so full booking him should be no problem at all. The first set of masteries should be achieved with gems depending, if you don't have gems just farm them.

Save the Legos for down the road when you are later in game and know what holes you need to fill in your teams. It is still always best to six star your starter champ IMO.

I'd probably go Ninja. While Kael is a nice enough farmer when maxed out you will inevitibly get better farmers as the game goes on (I might not be the best judge of that though since I've been using skullcrown for the best part of a year despite having much better champs now) whereas Ninja will likely see a lot more use for a much longer time.

Do you want a good campaign farmer now who you will replace in a short space of time or a dungeon boss/CB killer who isnt so easy to replace?

Perfectly said. I would go Kael as well. He's such a strong champ he will be with you well through the midgame.

Ninja would be my second to 60. He is amazing, and I use him all the time myself. But as a legendary, he is MUCH harder to book due to the scarcity of legendary books.

if you are f2p I would go with Kael, if you plan on  spending money on the game build Ninja or one of the other legendary champions!

If your 50 Kael already can do 12-6 on brutal, you've done a good job so far. Depends on his time to clear the stage to make him or another of your champs 60. You can use Kael in many other places in the game, so you're doing nothing wrong with him, even if you've got Banshee as a better poisoner. As a farmer Bellower is much better. Kael is way better at clanboss which needs other artifacts than a single farmer needs unless you've got a uk team.

Check your artifacts. If you have some more lifesteal artifacts and can speed up Bellower to approx. 160 and will get him to almost 100 kritq and perhaps 150 (better more) kritd you could try Bellower. Perhaps that would be good enough for 30 to 45 second runs, don't know. I was really long farming stage 12-3 with Elhain which really was chewy (1 minute plus). After some months or so I've got Bellower (atm at spd 221, kritq 100, kritd 238, atk 3000, no lifesteal, helmsmasher mastery) who does it much faster, though the last wave always needs two hits (better was 250+ kritd as already mentioned above by another writer). Hopefully that helps.