At the current point in comic (early stages), Owen just discovered his powers so he's pretty much a noob. He's an average, geeky 12 year old boy, physically the weakest and lacks combat knowledge. His greatest setback is that he lacks confidence due to his sheltered upbringing. Most of his abilities manifest through flight or fight response.
He represents the state of matter Gas. Throughout the series he serves as a scout and assist for his comrades and it's only in later moments he gains two offensive abilities. His deadliest being a "Disintegrate" ability.
In the matchup below, we'll pretend he has better control of his abilities. In the comic he can fuse with his comrades (like a Materia in FF7) to assist them in augmenting their abilities. Leaving out his assist abilities and treating below as if he's on a solo match:
GOOD MATCHUP:
He can easily evade physical attacks by shifting into his gas state. He can avoid grounded enemies by levitating (his particles) or sneaking around through porous spaces. He has a flash-step and teleport which can help him confuse the enemy. A flash-step + physical push can help him knock an enemy off a cliff etc.
He can levitate his camera and move it around independently, to further spy on or distract enemy while he attacks them from behind.
If an enemy stabs him, burns him or chops a limb off, he can regenerate once he switches into gas from. As he returns to human form, his body resets to how it was when he first heard the 'Hum' that activated his abilities.
This also goes for his camcorder too, which was on him at the time of the Hum, it can regenerate if destroyed.
(Aging only has effect on his human form, if for some reason he never transformed into gas in over 30 years, he would age naturally to 42 but the moment he transforms into gas and back to human, he will reset his body to age 12).
Fire Elemental Enemies: In gas form, extreme heat causes him to speed up and form plasma. He will remain unharmed and can use his plasma state for offensive attacks.
By default, any mammal based lifeform that isn't a "Sick Boy" and stands within a 20ft radius of him will automatically mutate into Xynous - a form of infected that acts on base instincts and the need to feed/infect others. Xynous are constantly shifting between the states of matter -Liquid, Solid, Gas at high speed, an experienced Owen can mentally alter them to a preferred state of matter eg. Solid, and disintegrate them or kill them by physical means.
Once he earns someone trust he can teleport to their location, so if somehow he can earn an enemy's trust he can teleport to wherever they are. and paired with the skill below is a deadly combination:
His disintegrate ability works against particle based organisms and spirits (he learns the skill from a figure who lives outside the states of matter)
This ability takes two forms:
(1) A ranged/projectile form that partially dissolves an enemy (lower willpower)
(2) A complete dissolve: he must latch onto the enemy in order to do this.
Willpower is key. An enemy with lower willpower will be quickly dissolved but an enemy with greater willpower is immune by default. He must either outsmart the enemy or earn their trust in order to latch on and dissolve them. The greater the enemy's willpower the slower the enemy takes to dissolve.
(Sick Boys are asymptomatic carriers of a virus that mutates others into monsters called Xynnous. Once the victim stands at least 20 feet within range of the Sick Boy, they will instantly mutate [some Sick Boys have longer range]. Sick Boys also display a variety of superhuman abilities.)
BAD MATCHUP
Face to face he has no chance, his best bet would be to teleport out of there
Owen's Human form is Mortal but his gas form isn't. Anyone who can rob him of the opportunity to completely transform into gas can kill him. His Gas form is is true asset, allowing him to break the shackles of mortality with that regeneration (mentioned previously).
Ice elemental enemies can slow him down, cause him to condense into liquid and then freeze his particles into a solid icicle. Because he would have been a gas for this to happen, he would remain alive like this for eternity, until someone casts sublimation/melt.
Mind corrupting enemies: has a greater effect on an inexperienced Owen. When he first started out, he had a bad dream about his particles dispersing and out of his control. His thoughts has a major effect on his power, once he allows such thoughts to cloud his mind, he will start to disperse and be forced into human form (leaving his human form vulnerable). A Sick Boy with this ability exists in the comic.
Extreme Wind Enemies: they can blast his particles away, causing them to disperse and force him into human form. There's a father/daughter pair in the comic with Windstorm abilities.
Vaccuum type enemies: they will suck his particles and (1) force him into human form (2) consume and absorb him. Some species of infected (Xynous) have this ability. Examples include: Infected bats, walruses, seals, whales and those mixed with any combination of these animals.
Gravity/ Anti-Gravity - There's a major Sick Boy character named Issandro who uses these. With his gravity fields he can rob Owen of all independent movement, bounce his particles around or lock them in place for eternity. Issandro is also a skilled martial artist so he can read through Owen's amateur strategy easily.
Anti Power - A Sick Boy with this power exists in the comic, he can counter Owen's abilities with ease. The character is a skilled wrestler with herculean-level strength and defenses. Once he counters Owen's abilities, the 12 year old would be forced into human form and dispatched within seconds. He has better flight experience than Owen too
Negation/ Regeneration Feeder - one exists in comic. This character can suppress Owen's gas abilities via Negation fields. The character hunts and feeds on other Sick Boys with regenerative capabilities to continuously replenish his own_(think Jeepers Creepers)_ If this character were to dismember Owen's limb etc Owen will permanently remain this way even if he shifted between gas and back to human.
Got carried away, now looking at the time, was so excited to give this a try, hopefully this makes sense XD
Running off to bed, enoy your night everyone lol