Halcyon Spam: As far as i know this strategy has only been used against the covenant, but would probably work in UNSC vs UNSC. Set the frigate factory rally point of two or more planets to a single enemy planet, then construct as many Halcyon classes as possible in as steady of a stream as possible, the high HP and low cost of the Halcyon's will ensure that the enemy defenses will be inadequate to defend the planet alone, therefore the enemy fleet gets locked to a single defensive position, allowing openings for flanks or sneak attacks.
COV vs. UNSC: keep in mind this only really works if you know when and where the UNSC are going to attack in full force. Move all of your fleets to the leading edge of the gravity well, when the UNSC jumps in, they will be facing the other direction and won't be able to fire MAC rounds on you immediately, covenant ships will still be able to fire plasma torps and energy projectors at point blank range. If performed correctly, you will lose little to no ships, and the UNSC forces will lose all of their invading army likely before your patrol fleet even manages to get to the battlefield.