Battle of Ilum Flashpoint Guide
Battle of Ilum First Boss - Gark the Indomitable
A very simple tank and spank boss. Really the only important thing you need to know about him is throughout the fight he will spawn some adds which the tank will need to quickly pick up so they don't own any DPS or more importantly the healer. The adds only have about 5k HP so they will be extremely easy for your group to AoE down, just have the tank drop whatever AoE they can
Battle of Ilum Second Bosses - Velasu Graege & Drina-Zel
For this fight you will be fighting two bosses at the same time, Velasu Graege and Drina-Zel. Velasu is a ranged boss and Drinda is a melee boss. The tank will need to grab both of these bosses and keep agro on them throughout the fight (duh!). From my experience with these bosses Drinda has really spotty agro; it seems extremely difficult to hold agro on him even when he isn't the one being dps'd first. He very commonly jumps to the DPS or Healer throughout the fight at which point the tank will need to taunt him in order to regain agro on him.
I recommend you burn down Drinda first for this reason, keeping agro on him is more of a chore than anything so I make sure you save your taunts for when he jumps away from you. During this fight when you get one of the bosses to half health (or somewhere around that time) 4 turrets come out around where the bosses were standing before you engaged them. The DPS will need to burn these turrets down as they will start shooting anyone in range causing massive amounts of damage to be taken by the entire party.
Battle of Ilum Third Boss - Krel Thak
This is probably the easiest boss of the instance (aside from the very first one that you face). Some groups have trouble with him but I have never wiped or had anyone drop below 50% on this boss (at least on normal, on Heroic he apparently has this nasty Damage Shield ability that he uses. I'll explain it later). The main ability that you will notice from him is the adds that he summons. Four adds will run out at you during the fight every 30 or so seconds which the tank will need to quickly pick up. Just before these adds come out the boss will stun the entire group for a few seconds making it more difficult than it should be for the tank to pick up these adds.
If the adds get on you do the tank a favor, run towards him and let him pick them up, don't run away from him. Aside from the adds Krel also will target a random player of the group and start blowing them up with his high damaging abilities. Have the tank taunt and the healer to spot heal whomever is taking the damage. The other ability he does, the Proximity Probe, as aforemetioned I think is only used on Hardmode as I have never before seen it on Normal.
Proximity Probe & Defense Screen: Apparently this boss uses an ability called Defense Screen on every other wave of adds starting with the very firsy wave. This ability can be interrupted so you will want to do that. If not, it makes him immune to damage for a short period of time and if anyone attacks him while Defense Screen is active he will counter with an ability called Proximity Probe which will knock the player back and deal a great deal of damage to them.
Aside from that this boss is cake!
Hardmode Tip: Have everyone in the group use their CC breaking ability at the same time during one of his mez abilities to burn the boss down as quick as possible as he has a short enrage timer. He will mez 4 times during the fight which means the group will miss a total of 30 seconds of DPS if no one breaks this CC.
Battle of Ilum Fourth Boss - Commander Folek
This is a mini boss right before the final boss of the instance. It's an extremely easy tank and spank fight. Just eliminate the adds before focusing on the boss.
Final Boss of Battle of Ilum - Darth Serevin
This is a really easy tank and spank fight for the most part he has a few moves which I will get into later. He comes with an add which you will want to immediately take out. Once the add is elimated it will be just your party and the boss. Throughout, phase 1 we will call it, he only has one special move which involves him picking up a large crystal shard and throwing it at a random party member. It does upwards of 4k damage, not much to worry about just heal through it.
After this is "phase 2" where Darth Serevin will vanish and appear next to a random party member using a force choke like ability on them and also using the crystal shard ability aforementioned on another player. When he vanishes it resets his agro so the tank will need to quickly pick him up once he reappears. That's all there is to this fight!