Reduce the cost of Guardians
To allow advent to be able to field enough of them earlier and make their losses in the endgame easier to replace.
Repulse is completely counterable, and honestly I don't think it justifies its position in the tech tree nor its cost, since everything outranges it nowadays. If there was an upgrade to allow repulse to outrange everything else again, it would have a justified cost.
But as it stands, with all the micro involved it can SOMETIMES be effective. considering guardians already are slow, the balancing element already exists. Also, considering guardians are pivotal to the advent fleets effectiveness, having them difficult to acquire weakens advent as a whole.