this is the most powerful gigas in WKC 2, They both look the same there are some slight differences.
One is their colors...
Duke = Red
General = Blue
Also most of they're attacks are the same, however only General Gigas summons 4 shadow soldiers.
Though they are powerful, they are not impossible to beat.
They have two forms, Beast & Warrior
In beast form, just hitting it with a basic attack would only do about 10 - 50 damage, and stronger attacks only do about 500 max, don't bother doing this. Instead, when it starts charging its basic attack, 1 of the 4 eyes on its body will start charging a purple color.
Simply hit the charging eye with the basic attack (Example: Slash for sword users) only once, then wait for another eye to start charging.
While its in beast form. Hitting the charging eye would do about 400 - 800 damage with just the basic attack.
If its whole body starts charging like a dark red color, either block (defend), or get out of its range.
As far as General Gigas goes, if it stretches straight up and starts glowing slightly dark red, its summoning shadow soldiers (4). just kill them quickly, then get back to the beast.
After hitting its charging eye enough times, it will transform.
Once in its warrior form, it will be weak against slash attacks.
NOTE: Attacks that cost Mana / AC may do more damage than basic attacks however it will not make the duke / general gigas transform any faster, and even if you manage to get its health down some it would only go back up once it transforms. I'd recommend using only the basic attacks (explained above) while its in beast form then once it transforms then use stronger attacks.
This way you save on AC & Mana in case you have to face another boss (example: Gatekeeper Quest).