can i play guards during a shodwon to a bf i am defending and control

Yes — but only if you play it from Hidden; Guards! from hand can't be played during any showdown.

Guards! has no Action or Reaction keyword, so from hand it's limited to your own Main Phase in a Neutral Open state (Rule 343.1.a) — being the defender who controls the battlefield doesn't change that.

If Guards! is already hidden facedown at a battlefield you control, it gains Reaction (Rule 811.6), so you can play it for [0] during a showdown on any player's turn, including a combat showdown at a battlefield you're defending and control. In that case the 2-might Sand Soldier token must be played at the battlefield where Guards! was hidden (CR 811.1.d.5) — which is exactly the one you control, so it works. Two caveats: the card has to have been hidden on an earlier turn, and Hidden requires a battlefield you control, so you couldn't hide it at a battlefield you're attacking.