When playing Sauron,
I have trouble stopping Beravor
(the "ranger") from concluding both Quests... They seem too easy & within reach. Even if I get enough Influence to spawn Monsters (which I can only Spawn 1 per Monster/Minion Order, if I recall correctly, & the Minions are either too far away or are too slow), he always has other paths to choose & circunvent 'em. & most Influence wouldn't help make locations perilous for him either - he has too much Wisdom!
My worst case scenario: Beravor's Mission Card: To complete all his Quests(!)
If she has the Grey Havens SQ, I'd say don't even bother, hope the Crebain get a miracle draw (Block) and stall her out, making her waste a turn. Focus your efforts on getting Plot requirements into play, maybe blocking path to Angmar. Redhorn Gate is a bit easier, you should have influence at Gladden Fields from your Starting Plot, if not, take one Influence Action and make a chain to RG, then place a monster token there. Yes, those are hit and miss (40% blanks), but if she has slightly bad moving cards, meaning she has to pay the number of cards for any one more (instead of just discarding the right symbol), she should be low on card by the time she hits RG, maybe enough that she doesn't manage to kill the monster.
Her Advanced Quest, placing a monster before Ruins of Angmar is something I don't think I've ever done TBH. The Purple stack isn't one of my favourites, monster in it are fairly weak. Best bet, corruption at RoA, then the 20% luck of the draw that you get the RoA-specific Peril card which ends her turn right there. You won't stall her until Gothmog arrives, but if you happen to get her slowed down in the north decently long, you might get Gothboy to attack her.
Granted, 1-hero games make it tougher due to the two turns back to back for the hero. However, one plus side for Sauron is that Beravor starts the furthest away from your general Plot areas, make use it this, you know she won't be taking off your influence in Rohan/Gondor until about turn 3 probably.