About every 30 seconds a glowing red glyph will appear on the ground near you, which gives you immunity to the blast damage that goes off 3 or so seconds later. The easiest (albeit cheeky) strategy in this fight is to stand over in whichever corner is furthest from Samael and wait. Spam this skill anytime you take damage to get back up to full HP quick.
Something sadly not mentioned anywhere in-game or in docs is that the poison skill Harvest regenerates more life the more hard points you put into it- ~20% life per second at high levels, which is enormously helpful here and with the Witch uber in particular. Socket On runes in weapon and Perfect Onyxes/Perfect Ambers in armors to 90% res, the rest can be On runes or %dex sockets to reach 3000. Surviving Samael for 11 minutes requires 90% max cold and light res (you might get away with only 85% cold as only worms do cold damage), 20K+ life, high %PR and a good source of sustain. Cubing PWC with a Taha Rune and Greater Signet of Learning gets you 1 ticket to Sam.
Getting to the fight requires the Pulsating Worldstone Crystal: cube the Pulsating Worldstone Shard (rare drop from Hell Baal) until it stops changing, then combine in cube with Assur's Bane, Ring of Virtue, Ghost Trance, Glorious Book of Median, and Neutrality Pact.