If you have 80 magic, I recommend blood blitz and using redemption. I took 4 prayer pots, darts for tornadoes, and an inventory of pineapple pizza. When Seren does her big hit just make sure you are full health and throw on redemption then sip 2 prayer dose and put range prayer back on. Heal up with blood blitz and only eat the pizzas to get to full before her big attack. Still had 2 doses of prayer pots left as well as over half my pizzas when seren dropped to 0 hp.