I got the perfect person for Isaac to fight!
Meet Double from X4:

His TRUE form (AKA His Battle form.)

As his name suggests, Double has two forms: the first is a stocky, overweight, amicable form, but his second and true form is a taller, slimmer, and stronger battle-ready Reploid, with each hand equipped with energy sabers and strangely a gelatinous substance makes up about 50 percent of his body. His overall character is also shown to be significantly insane upon revealing his true allegiance, breaking out into maniacal laughter, frequently shouting phrases that involve destruction of some sort when attacking X, and has little, if any, remorse for his actions, and in fact mocks X for his trusting nature, stating that it would be his fatal weakness. He also implies that he might also consider betraying Sigma in the future, as he states that among the groups he declares idiots as they made his job easier, Mavericks were included in it. He's immune to electricity, and cutting weapons due to his mostly gelatinous form. Also if you were to hit him with a weapon he's vulnerable against small pieces of his body comes off and will attack his opponent.
His theme:
http://www.youtube.com/watch?feature=player_detailpage&v=IvtgBBW_vM8