What's the trick for a PC Ranger?

The Achilles heel of the Ranger is its relatively low defense. Swarming it with lots of fleets of pure offense ships is the best approach. Use small hulls and fit them out with nothing but weapons. The most difficult part will be to get the first strike. If the Ranger strikes first, your fleets will be toast, so try to lure it in with a decoy. If possible, try to use military starbases to give your fleets an additional boost. In any case, depending on your tech level, you will lose many fleets, but the Ranger will be eventually worn down.

Note that the approach suggested above probably requires you to have a high logistics ability.  That is, you need a lot of small ships in that fleet.

Two other approaches.  One would be to make peace with the Drengin.  With them at war with the rest of the galaxy, this might be possible.  Once at peace, you might even be able to buy it from them.  The other would be to eliminate the Drengin.  That ship cannot defend their planets from invasion.

A third approach would be to stick a ship or fleet faster than the Ranger near it and wait for it to be targeted by the Ranger.  Then lead it around the map by its bowspirit for as long as you need.  This approach wold generally require a high enough intel level on the Drengin to let you "see" the Ranger's destination/target.

Thanks for the tips.

I've also considered luring it (as Gaunathor suggested) away from the Drengin home planet then taking that home planet by invasion before the PCR can get back to defend it. Right now I have several troop ships near the home planet but out of sight of the PCR. Wish me luck! Since it takes as much distance for the PCR to get to a single ship of mine as a fleet, I'm going to bring in several single ships widely spaced to keep the PCR running from place to place.

I've noticed that the Rangers appear for civs that are behind in the game. It keeps taking over the universe from being a slam-dunk even if I'm way ahead.

LTjim - I've noticed the faint trace showing where the enemy ship is heading but didn't realize that came from high intel. I don't know at what point it kicks in, but I try to get four spies on civs I think will be attacking at some point.

You generally should also see its destination or target named, though not always.

You need 'high' knowledge of a race to be able to know ship destinations; it takes four spies some time, maybe a couple months, to get that, but once you have it you can repurpose the agents.  

I once had this ship come up twice in the same game, for the same race. This same race already owned over 60% of the galaxy (and growing, I had about 10%) and had the largest and strongest fleet in existence. Dragging victory out of that fire was a nice challenge.

Point is, it doesn't necessarely appear for the failing civs.

As far as the intelligence thing goes, you need at least as many spies as the intelligence level to achieve that level. "High" intelligence is the third level of intelligence possible, so you need at least three spies on that civilization, though four might make it go faster.

As for your current situation, given that you seem to be able to outmanuever the Drengin, the best strategy is LTJim's option 2: avoid directly fighting the Ranger and go after the Drengin planets to conquer them or force them into a surrender.