This skill would be too broken if it is consistent."
That's not the entire point. Balancing a skill is fine, but don't make it ridiculous.
Even if you managed to pull off this skill, there is no guarantee you'd be able to make use of it when you really need to.
Ojama go is easier to use and in most cases during a match it's just as good as this skill.
You can use mausoleum of tge emperor to summon gandora and use his effect twice by setting a spell/trap in order to get to/below 500 LP and clear the opponents monster board. However one E Con is enough to ruin your strategy