Spell Skills, like Boomerang 1-40, increase damage +2.5% per Rank (except for a few Rogue Spells) plus three other things get boosted for that one Spell. many Players just do Spell 20 for +100% damage and the other boosts, so 2x damage for that one Spell. this applies to all attacks with that one Spell.
Elemental Crit Damage +100% applies to all attacks affected by that particular Element for the duration of the Elemental Critical.
if you have Boomerang 20 from Hero Skills and are happy with that, and want the +200% Frostbiting, then you are getting 2x damage from the Boomerang 20 (even more damage if you are piercing more than one enemy at a time) and then 3x damage from Frostbiting for as long as the enemy is affected by Freeze (3 seconds). enemies can be Freezed up to 5 times? but I have discovered that if you are on a high floor and are using low damage Skills, you can cause more Elemental Crits after a few seconds have gone by, at least I was able to do that with Fire and Immolate. haven’t really tested it with all Elements.
the trick is that you want Celerity to speed up Boomerang so that you are making more use of Frostbiting, and that Talent that increases Damage when your Boomerang returns.
you could always go Boomerang 40, Ice +200%, Frostbiting +100%, which gives 3x, 3x, 2x instead of 2x, 4x, 3x…hmm, in the first, you do more regular damage but lower Frostbiting, and the second is lower regular damage but higher Frostbiting (9x, 18x damage vs. 8x, 24x damage after Frostbiting).
hope this helps and not to confusing.