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.

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.