It has been a while (pre 5.0, so 65 was the level cap) )since I did Heskal, but I remember the fight being long and irritating rather than impossible...

There is another thread about Heskal, here which gives some basic advice but no scripts for defeating Heskal. Basically, it amounts to "make sure you have gear which is up to your level, use your interrupts when you need to, kill the additional spawns as they appear, and make sure you are running the chapter on story mode, not veteran".

For gearing, you could leave the story alone for now and run heroics and flashpoints for a while to make sure you have level-appropriate gear.

If you need to change the difficulty level, having selected the harder Veteran version at the beginning, I do not think resetting will do the job - you may have to abandon it and pick it up again. Also, getting help from friends should be possible in the veteran version, afaik?