Another tip: After eliminating the screechers, before/as the seekers all run to the boat, talk to Gareth. You can get him to lend two seekers to support you in the final battle. On my play through, they were pretty weak offensively, but served as useful distractions so my party didn't get as wrecked by the Alexander.
And as others have said, high ground and spacing your. Party out is great. I managed the fight from the top of the stairs in a straight ahead approach, but created an oil field at the bottom to slow Alexander's advance and set up a fire death trap. Preparation is key.
