Magicked Barrier only reduces magical damage by 10% but most tankbusters are physical while most raidwides are magical. The physical ones are generally "fairly" easy to spot and are things like stomps, tumults or dashes. Basically things that give off a brute force feeling.
So using them on raidwides is the best and safest option and because healers generally have to heal the party more than the tanks as well, so you're on the right track there.
Some examples from the current tier would be Ruby Glow, Venom Squall, Hemitheos Fire, Spark of Life, Genesis of Flame etc.
If you're doing fights multiple times it's also worth noting which ones hurt more than others, e.g. Ruby Glow hurts more than Sonic Howl.

And it's always nice to see a dps wanting to use their mitigation correctly, props for wanting to get the most out of that skill.