Steadfast Armasaur — ruling 1

Tapping an attacking or blocking creature doesn't remove it from combat. If the target of Steadfast Armasaur's ability survives the damage, Steadfast Armasaur will deal combat damage to and be dealt combat damage by that creature as normal.

Steadfast Armasaur — ruling 2

If Steadfast Armasaur is no longer on the battlefield as its ability resolves, use its toughness as it last existed on the battlefield to determine how damage is dealt.

Steadfast Armasaur : règles, fonctionnement et interactions (MTG)

Steadfast Armasaur is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Vigilance , : This creature deals damage equal to its toughness to target creature blocking or blocked by it.

How it works : Steadfast Armasaur

Original English text — Wizards of the Coast / Scryfall

#1September 29, 2017· Wizards of the Coast
Tapping an attacking or blocking creature doesn't remove it from combat. If the target of Steadfast Armasaur's ability survives the damage, Steadfast Armasaur will deal combat damage to and be dealt combat damage by that creature as normal.
No other card is named in this ruling.
#2September 29, 2017· Wizards of the Coast
If Steadfast Armasaur is no longer on the battlefield as its ability resolves, use its toughness as it last existed on the battlefield to determine how damage is dealt.
No other card is named in this ruling.