Vorcha are considered vermin by other species inhabiting Citadel space, but their unique physiology and innate aggressiveness make them shockingly durable combatants.
A vorcha's resilience and malleable DNA allow them to evolve and adapt to any situation, making them formidable opponents on the battlefield. Their lightning-quick pounce is a direct result of their societally ingrained bloodlust.
The Bloodlust power can be triggered by any kill, not just melee kills. Each Bloodlust stack lasts only fifteen seconds, which forces Vorcha to constantly kill in order to keep the maximum bonuses that Bloodlust offers.
Flamer can be used and swung around to target multiple enemies, setting them all on fire. Considering it already has a range limit to it, you'll already be within close enough range for Cluster Grenades to be effective (accurate). Follow up with a single, maybe multiple, Cluster Grenade(s) to start killing enemies and simultaneously activating multiple Fire Explosions at once, for a potential Killstreak without a missile.
Choosing Cluster Grenade over Flamer enables a no-cooldown approach. This allows the vorcha to equip the heaviest of weapons, and so long as the Bloodlust power isn't deactivated, cooldowns are not a concern.
Cluster Grenades are useful against all enemies. Cluster Grenades have a more universal use, and are not restricted to range, but are less specialised at taking down armored foes compared to Flamer. They certainly do the trick against unprotected foes though.
Due to Cluster Grenade's splitting reducing their accuracy over range, and Flamer's range limit, one should either look into a weapon that contrasts their range limits to balance such, or embraces the range limit, and will support one knowingly walking into CQC situations.
The Reegar Carbine has massive DPS, but a low range-limit, like Flamer, and fits the latter role exceptionally. Combine with kills, and Bloodlust's regenerative properties, and one can become the organic embodiment of a tank.
Melee combat should be approached carefully. Vorcha can be killed when attempting heavy melee against stronger enemies or large groups. Misjudgment of health in enemies with instant-kill animations places one within lethal range for these attacks. That aside, it is among the fastest heavy melee attacks in the game. It closes distance to the target like krogan charges, though it is easier to aim at close ranges. Thus, between melee damage evolutions from Bloodlust and Fitness, one can move rapidly from target to target, shredding enemy infantry.
Flamer's high damage evolution can be used to kill groups of Troopers, Centurions, Engineers and other infantry very quickly at close range. Against Phantoms and Atlases, caution should be used since on higher difficulties they can easily inflict more damage than you can handle. It is against these kinds of enemies that you should consider staying close to team mates. Opt for a piercing weapon, or Piercing Mod on one of your weapons to make sure Guardians don't get the edge on you.
Cluster Grenades will prove devastating against groups of foes due to the high damage and clustering effect of the grenades.
Extra care should be taken against the Collectors, since they have both fast and powerful hitters at their disposal, specially since Vorcha have low shields. The heavy hitters will strip most of your health and the quick hitters will keep your health low while the harder hitters finish you off. If a group looks too strong, it probably is.
Cluster Grenades will prove decisive against standard footsoldiers and seeker swarms, but will serve little use against Scions, and will defeat a Praetorians barriers, but not their armor.
Flamer will devastate every foe except Captains, Possessed enemies, or Praetorians when their barrier is up. However, it should prove very lethal against every other enemy.
Flamer can be used to great effect against unshielded Geth Primes, as it does very high damage to armour. Otherwise, all Geth (except Geth Troopers) have shielding protecting themselves from any flame attacks. While capable of regenerating, the geth are "hard-hitting" specialists, and will drop vorcha's health quicker than they can regenerate especially in groups, so a "cautious but aggressive" approach may be best here.
Cluster Grenades can help in a pinch to spot Hunters, or dispatch bombers, but otherwise serve little practical use.
Flamer is effective against Reapers, due to the majority of enemies having armor, or just health. With the Armor Damage evolution for Flamer and both health regeneration evolutions, the Vorcha Soldier is capable of taking down Ravagers and Brutes. The green acid puddle that Ravagers release upon death also deals minimal damage to the Vorcha, provided that Bloodlust stacks are still active.
Again, Cluster Grenades will devastate groups of weaker foes like Husks, Cannibals, and even Ravagers and Brutes.