Changeling Berserker

Haste is a static ability.

If a creature has haste, it can attack even if it hasn’t been controlled by its controller continuously since their most recent turn began. (See rule 302.6.)

If a creature has haste, its controller can activate its activated abilities whose cost includes the tap symbol or the untap symbol even if that creature hasn’t been controlled by that player continuously since their most recent turn began. (See rule 302.6.)

Multiple instances of haste on the same creature are redundant.

Champion represents two triggered abilities. “Champion an [object]” means “When this permanent enters, sacrifice it unless you exile another [object] you control” and “When this permanent leaves the battlefield, return the exiled card to the battlefield under its owner’s control.”

The two abilities represented by champion are linked. See rule 607, “Linked Abilities.”

A permanent is “championed” by another permanent if the latter exiles the former as the direct result of a champion ability.

Changeling is a characteristic-defining ability. “Changeling” means “This object is every creature type.” This ability works everywhere, even outside the game. See rule 604.3.