My guild is attempting some heroic bosses tonight so I am here to look for some advice on certain fight. I would like to get some advice from fights like H Elegon, H Feng, H Blade Lord, and H Wind Lord. Thank you very much for your time.
There are a few things on your character you can improve easily with regemming. You should only gem for non-int when the socket bonus is able to make up the difference in lost Int. A general rule of thumb is to only gem for a socket bonus when the socket bonus rewards more than 80 Int.
As for fight specific tips:
You can Havoc the boss and stand outside the ring and do 1-3x shadowburns on the add (mostly wasted damage) and on the boss. I haven't personally decided whether it's better to Havoc a Chaos Bolt while you have the damage buff, or get 3 shadowburns on the boss.
For sparks you can spec Manno's Fury and easily get off 2-3 Incins/Conflag on adds. You can also havoc an add with chaos bolt.
The most damage destro can do on this fight primarly comes from the adds in shield phase. Pre-RoF + FnB Immo the adds and continue to incinerate/conflag them down. When they start to reach 20% health you should Havoc the boss and get 3 shadowburns on the boss from shadowburning the low health adds. You should aim to have 1-2 embers before you start to shadowburn the adds so you gain as many embers from shadowburn returns as you can.
H Blade Lord:
Not a great fight for destro because it's single target. Save your second pot for execute phase when you have all of your procs and get as many shadowburns off as you can. You can stand behind tornados for when he does his pull-in to not have to use a portal to get away. (It stops your movement) KjC is extremely useful on this fight.
H Wind Lord:
Destro can do some amazing damage depending on when you kill the second set of adds. You should keep FnB Immo + Conflag + Incins on the adds with 100% uptime on RoF. During recklessness you should use Dark Soul, keep RoF on the adds and just cast as many Chaos Bolts as you can.
If you get lucky enough to kill the second set and have the third set below 20% health for recklessness, you can do some redicoulous damage with Havoc'd shadowburns on the boss.
Beyond that there's not all that much beyond buffing your chaos bolts and shadowburns with as many buffs as you can when you use your embers.