Athiss

Did whole instance with 4 lvl 19 players. Written from the perspective of a BH merch healer.

First boss = tank + spank, weak adds
Second Boss = big rhino that does knockback watch out so you don't get knocked down the cliff. Adds will come so make sure someone get's them off the healer (they don't do a lot of damage).
Final boss = Tank + spank. He has one DOT he does on random party members that do a LOT of damage, so make sure everyone is topped off. During the fight he will disappear 2-3 times (depending on how long you take) and 4 flame orbs will appear and follow ONE targeted player, this player should kite them around until they despawn (shortly after the boss reappears).
Most importantly keep everyone topped off, cause that DOT will kill someone at half HP (at least at lvl 19).

Doing it at lvl 19 was very doable, but you need to mind the heat. Use time the boss is invisible to heal up everyone and vent heat if available. A medpack for yourself may come in handy.

IF someone dies during the fight and runs back in to help, the boss will reset at full health (happened when we did it, a DPS died from the DOT).

The final boss is basically Tank 'n Spank. Though through-out the battle he will stealth away and one of the players in the party gets a debuff which attracts 4 spheres of fire that you need to avoid. Unless you're prepared to take a lot of damage.

The boss fight took pretty long when I ran the flashpoint with a group and we were all around 23-25 ish at the time.