sejuaini killed my entire team with just her ulti and w?

Match history link plz.

Don't get grouped up so close with Sejuani. Positioning is important against tanks.

Full AP Sejuani has some serious burst to her kit if she lands her ult, so you have to watch your positioning. Her main weakness is her short range and squishy nature while her ult is down, so she's pretty much a one trick pony. That said sometimes cheesy builds work.