Phase Spider boss should be approached as level 4 IMO unless you want to drive yourself mad with savescumming. You sure can do everything with level 3 party, but it's like shaving with a sharp rock. IMO, it's just part of how things should be done, after all scouting ahead and picking your fights only make sense.
That said - you can cheese it quite a bit. If you kill things from stealth in one shot, you won't be detected so you can take your wizard/warlock and blast the two accompanying spiders there with Scorching Ray with a good 90+ accuracy because they are downstairs and you have high ground advantage and all. These two phase spiders increase boss difficulty a ton.
Once you deal with those two - the boss is a cake, you can just CC it to death as Battlemaster, but even if you fail - it's still one mob vs 4 party, sure summons spiderlings, but these are not really a problem.