How do yall build your Gareggas?
I usually keep about four end-game, two with officers, both Aggressive (Reckless would probably be fine as well). S-Mod Heavy Armor, Reinforced Bulkheads, and Hardened Shields, add Flux Distributor and ITU with OP. Breach SRM and Sabot SRM. I have a modded weapon on the head slot (Electron Rifle), but something Fragmentation or EMP would work. Your choice if it's slow and heavy or constant pressure.
Breach SRMs are great for softening up bigger targets due to their secondary property and are enough to finish Frigates/Destroyers if they're out hunting smaller targets. Sabot SRMs are mostly vulnerable on the way in, but since the Garegga is a knife-fighter they tend to go off as soon as they launch them making them really deadly. Even if they don't hit shields, the Sabot's EMP effect can cripple a ship, leaving it vulnerable to your heavy hitters. For the head I like Fragmentation just to take advantage of any open hull. Plus, Fragmentation tends to be pretty flux-efficient.
I have them Escort line ships at the start, then when everything settles out and the firing line is mostly stable I drop them off Escort and have them hunt anything trying to flank, or anything that's annoying. I build them tanky so it's pretty effective for me to give them some simple orders near the beginning and then forget about them until something small and annoying shows up. On my officer-piloted Gareggas, enemies rarely even make it through their shields, even on very long and very nasty fights (I'm thinking double Level 3 Vengeance Fleets).
To answer specifically how to handle Hammerheads and Sunders, I've never really had any issues so I'm not sure how helpful I can be. SILVERSWORD gives you a bit of a boost so you can catch them, and once you're on them, they've lost. Having a second frigate in the mix - or even some fighters - helps a lot since they suddenly can't just backpedal and beat you. If you're playing as a Frigate you want to take fights that 1) are low risk, high reward, and/or 2) force your target to choose between two bad decisions. If your target only needs to move in one direction to win without any downsides, it's a bad trade; you should back off and find something else. You're mobile. You can pick what trades you make quicker and easier than anything else, but if you over-commit you're going to explode.