Before the war, there were episodes where they'd fight the Dominion with just the Defiant and come out on top.
There were yes, but not all that many. Here are the main ones (since its a comparison between the Borg and Jem'Hadar I've discounted engagements with the Klingons or Maquis):
Season 3
The Search P1&2 – The first appearance of the
Defiant. The ship was under cloak and utilising stealth for the most part, but when she did engage the Jem’Hadar they did get their asses kicked, were boarded and captured.
The Die Is Cast – The Cardassian/Romulan attack on the Founders. The
Defiant is under cloak again and on a rescue mission, they aren’t focused on fighting, just saving Odo and Garak. They do take a few hits, etc but the Jem’Hadar are focused on the Cardassian/Romulan fleet.
Season 4
Starship Down – The
Defiant takes a beating, heavy damage and casualties, whilst they play cat-and-mouse in the atmosphere of a gas giant.
To The Death – The crew teams up with Weyoun to put a stop to a rogue group of Jem’Hadar. The
Defiant doesn’t see much action in this episode, but the crew are pitted against the Dominion on their own (though with help from Weyoun and his soldiers). The rogue group was never said to be all that many, and the crew to take losses and injured.
Broken Link – The
Defiant goes into the GQ alone to help Odo. They are surrounded by Jem’Hadar and know that fighting their way out would be suicide.
Season 5
The Ship – On a GQ planet, the crew faces a squadron of Jem’Hadar. No
Defiant in this one, but it is just a runabout crew. They do lose over half the team on the mission and another runabout.
By Infernos Light – The Dominion come through the Wormhole. DS9, the
Defiant and the runabouts are all that stand in their way (I’m not including Dukat as we all know what he did). DS9 was supposed to be far from the fleet, and though other ships would be heading out to assist, they are still very much alone and facing certain death. However it doesn’t come to blows.
Call To Arms – As the
Defiant mines the Wormhole (defended by the
Rotarran) the Station engages a Dominion fleet. They are massively out-gunned and out-numbered, but they pull out all the stops in order to distract the Dominion from the real battle at Torros III. Their enhanced weapons array proves to be more than able to handle the Dominion fleet long enough to complete their mission objectives before retreating.
After this, we are into the war and most (not all) of their engagements with the Dominion are as part of a fleet.