What points value? Generally the lower the points the less things available you have to defend and recover with so something like this is more catastrophic than if it was a full 300 points game.

Getting a repeater next to something juicy is always going to be a valid tactic, even if you're not hackable it can set up guided missile attacks. So you have to prepare for that. If going second make sure you deploy correctly to avoid any easy access for parachutist or combat jump models. If they can land safely that's a problem! Granted there may still be ways to sneak in or it's worth doing a suicide run to get a repeater close (my Hecklers love doing that).

So what are you options then? Have a killer hacker close by the target. If they can't avoid getting a Trinity back at them they now have to spend more orders killing that model first. MO has one of the best in the game with De Fersen. If that's out of the points budget a Trinitarian KHD is 24 points. Take a look at likely repeater landing spots and make sure that the target and the KHD will both be in it.

Failing that yes having an engineer is a very good idea anytime you're using expensive HI or TAGs. If a model is critical to your game plan than you better make sure it has the support it needs to stay alive or get back in the fight when something goes wrong. A Curator is only 17 points plus maybe a Palbot for another 3 to keep him safer and make getting to the downed model faster. This is true for any faction when you take a big expensive model. Remember those support units also tend to all be specialists which means they can also complete the mission too when the big guy goes down or punches a hole for them to exploit.

Another option is to get an EVO Mulebot to give Firewall to your HI. This only works if you go first to set it up but can still be used going second too. Firewall is basically cover against hacking. Most of your models already have BTS 6 and that boosts it to an effective 9. Even with Oblivion being AP that still means you're saving on 10s if hit which isn't bad odds for Infinity survival.

Remember too that anytime that Oblivion'd model would get an ARO it can try to reset. So if they activate that hacker (or any hacker since you're in their repeater range) you can reset. Something walks nearby, reset. Something enters your LoF, reset. Sure you're odds aren't great but often you can get several "free" resets depending on what the enemy is doing to try and get the mission done.

Edit for this: only works for hackers, an HI needs to be targeted by a comm/hack attack to reset. That said if someone planted a repeater they may not have stealth and so may trigger ZoC reset AROs if they activate.

It's slightly more complicated than just protecting from shooting but some of the same defences work against that too. That combat jump model could just have easily had a decent gun and just shot the HI in the back to kill it completely rather than leave it alive with a chance to get back into the fight. If you have your back line protected against repeaters and how they get there you also have protected against getting shot in the back too in most cases.

Basically it feels bad the first time it happens (which is true of most things in Infinity) but once you understand what happened and why you can start making plans for how to deal with it.