Chaos has a lot of very flux-efficient strong creatures which is fine as a way to distinguish it, but this one seems too good. Compared to other one domain creatures it has swim, swampstride, and undim when damaged, basically for two flux. The ability to undim makes this very powerful, especially in conjunction with two damage at base. I think the undim idea is a cool one so my idea would be to lower the attack by one. As a side note, while swim isn't superpowerful it does make this creature harder to kill since things like teleport and sunburst don't work.

You have to attack Archosaur while undimmed .
It is powerful, but you may see the cost: 5 flux. Any 5 flux 2 domain creature will kill it in a single hit.
If it had 1 attack, it would be too weak for its cost. It is fine as it is. Powerful but not OP.

Depends what it is against. Sure, if you can do 3+ damage it only gets 1 attack, but Sylvan that do 3+ damage are rare.

Against Sylvan 1-domain creatures it would do well even with 1 attack as it's going to take at least 3 of them with it (Treeherd excepted and it would still easily beat the Treeherd). That averages out to nearly the flux cost of the Archosaur and 2 more cards than it. At 2 attack the same is true against almost any 1-domain creature.

Swim also allows it to destroy shore buildings while being invulnerable to any non-ranged, non-flying/swimming creatures.

Suggesting you attack it while undimmed is ok, but it can still counterattack like any creature then. Basically what that means is unlike most other creatures who have to stay undimmed to attack, the Archosaur can charge ahead and then attack if it is attacked, possibly multiple times in a turn. That is a huge strategic edge.