All you need is 1 highlevel char and the twink you want to powerlevel.
Form a group, enter a Flashpoint for the twinks level.
When you kill any mobs the twink will gain next to no XP (quite a lot less then e.g. in WOW, 5-19 XP for a level 25 when 130k XP are needed).
The way to go: Let the highlevel bash a boss to like 2% of his life and then leave the group (be aware: it takes 30 seconds till he is really out, so just leave @ 25% or whatever fits for you)
Kill the boss with only the lowlevel in the group => full XP as for a solokill. For a level 20-25 this means ~10% of a level per boss.
To slow for you? - There are several bossencounter with more then 1 Champion in the game, just leave 1 alive and wipe, the encounter will reset and you may profit again, without any trash. Best Encounter for this is prolly the first Boss @mandalorian Raiders, just bash the 2 Dogs to like 1%, leave with the highlevel .. kill the Dogs with the twink .. earn 15%-20% of a level from 21 (may also work @20 alrdy) till 28. Takes me 4-6 Min with playing both chars (lvl 45 assasin, lvl 25 Bountyhunter).
Important: There are several Encounters with 1+ Bossmobs having random aggro, e.g. the mentioned boss @MR, just stay far enough away and in most cases this will solve the problem.