Wakka will come one in the second half regardless - you just had bad luck
timingwise. The game is quite winnable. Here's how to do it:

Firstly you need the Jecht Shot. If you didn't get it, you can defer to the
Sphere Shot but it will make life a LOT harder.

So I'll assume you have it.

Secondly you MUST use Manual (A or B is your choice). With Auto you can't
possibly develop any kind of spatial strategy.

The general aim is to get Tidus with the ball as close to the goal as
possible (like point blank if you can), and with not more than 3 defenders,
ideally 2.

The Jecht Shot will knock away two of the defenders, so if faced by 3,
you'll have to hope. Then as long as you're CLOSE to the goal, you will
score.

A good general strategy is to put Tidus as RF.
Get posession. Switch to 'left side formation' and head with your controlled
player down the RIGHT side. Tidus should then end up roughly central. When
he's fairly near the goal, pass to him, then shoot.

In general you need to score at least once in the first half, and ideally
again in the second BEFORE Wakka comes on. Wakka is not as good as Tidus.
Once they are switched you will probably not be able to score.

And don't concede.

It might take a few tries but it IS winnable.