okay numbers time
To kill Red Pirate on Legend you need to deal 1278450 water damage.
With dual Sieg you have around 4400 water attack and x27 in leader bubblie multipliers. You then need to match at least 5 x 3 water orbs to ohko red pirate.
With Sieg Isis assuming you run Sieg, Echidna, Chu, Bubblie, Midgard, Isis you have around 4000 water attack and x36 in leader/bubblie provided you match all 5 colors. You then need to match 1 of each att for Isis and 2 x 3 + 1 x 6 water orbs.
So with Isis you need less water orbs but need to match other atts as well so it's up to you.