How do you stop this combination?
Assuming that everything described in the scenario resolves, and nobody does anything to stop it, the player with the combo wins. Once the copy of Wild Slash resolves, the triggers play out as follows:


Arcbond targeting Seeker of the Way triggers. Seeker of the Way deals 2 damage to you, your opponent, and Zada. Since Seeker of the Way has lifelink, you also gain 6 life
If either player has 0 life, they lose the game.
Arcbond targeting Zada triggers. Zada deals 2 damage to you, your opponent, and Seeker of the Way.
If either player has 0 life, they lose the game.
Goto step 1.


Throughout this sequence, the original copy of Wild Slash is sitting at the bottom of the stack, waiting to resolve. So, even if it was possible to counter a triggered ability in Standard, that would be insufficient to stop the combo unless the opponent could do so twice.

If the opponent controls Tainted Remedy, then the last part of step 1 would instead say "you also lose 6 life", so in most cases, this would cause you to lose instead of win.

This sequence can be stopped at any time by removing one of the creatures, either by exiling it (e.g. with Complete Disregard), or by returning it to its owner's hand (e.g. with Force Away), or even by flickering it (e.g. with Eldrazi Displacer).

Stepping back further, as you mention, the combo can be broken by countering either the original or the copy of the Arcbond or the Valorous Stance, or by destroying one of the creatures in response to Valorous Stance.