Steel Hellkite's last ability destroys only nonland permanents whose converted mana cost is exactly equal to X, and only those controlled by players who have been dealt combat damage by Steel Hellkite this turn.

It doesn't matter who controlled those permanents at the time Steel Hellkite dealt combat damage, or if those permanents were even on the battlefield at that time.

You may activate the last ability even if Steel Hellkite hasn't dealt combat damage to any players that turn. If you do, the ability won't do anything.

If Steel Hellkite's third ability is activated with X equal to 0, it will destroy each nonland permanent with converted mana cost 0 the appropriate players control.

The converted mana cost of a permanent is determined solely by the mana symbols printed in its upper right corner, unless it's copying something else (see below). The converted mana cost is the total amount of mana in that cost, regardless of color. For example, a card with mana cost has converted mana cost 5.

If a permanent is copying something else, its converted mana cost is the converted mana cost of whatever it's copying.

In all cases, ignore any alternative costs or additional costs (such as kicker) paid when the permanent was cast.

If the mana cost of a permanent includes , X is considered to be 0.

If a nonland permanent has no mana symbols in its upper right corner (because it's a token that's not copying of something else, for example), its converted mana cost is 0.