Get the Medusa Head Soul from Subterranean Hell and activate the Malphas "Double Jump" ability. At the peak of your double jump, hold R to use the Soul and press Down + B to perform a downward kick. With R held, you will stay afloat yet gain speed as if you were actually falling from a high peak. This combination can deal one (or two) hit KOs to many strong monsters (for example, Treant or Alura Une) when R is charged, and can take down powerful Bosses in as few as five hits.
Alternately, using certain bullet/guardian combos can yield incredible results. Using Bullet Souls in conjunction with a high-level familiar can make for a powerful damaging combo. Other combos that yield great results are:
Frozen Shade / Quetzalcoatl [E]
Create blades of ice in front whelk Quetzalcoatl goes in for the kill. This is effective against single-file enemies.
Malachi / Gergoth [E]
Throw Malachi's shadow orb to continually damage your foe while shooting Gergoth's beam.
Succubus / Catoblepas [E]
Freeze your opponents in stone, then do double damage while regaining health.