He isn't weak to any particular spell but, as RuneScape gold he is wearing bulky melee armor, has a very low magic defense Flech can strike you with a crossbow that hits faster and harder than Karils. It can be 100% shielded by protect from range, however, if a bolt would of hit you, it will drain 1 prayer stage from you.
Just to make matters all the easier for you, Flech will alter her dhide to make her invulnerable to a specific type of elemental magic (atmosphere, water, ground fire). Flech will not change dhides until she is attacked once by a different kind of component (she won't change the first time, but she won't alter if she is not hit at all). Flech needs to be murdered with an elemental spell.
Flech will fall a variety of types of dragonhides, and arrows/bolts. She might drop her unique resistent dragonhide, along with her'unique' bolts. The sword is tipped with a very powerful poison that strikes 12s consistently. She can be attacked with any charm, however special spells like crumble undead, charge, etc.. will splash rather than hitting hard.
She will occasonally throw exploding potions at you, which will either hit 25 or 0. Anticipate at least one on your fight aganist her. Her armor is very lightweight, making her all the more difficult to strike.
Veneno will drop a variety of items from rune to dragon such as the DRAGON SCIMITAR along with the DRAGON CHAINBODY. The chainbody is very rare, a lot less probable than KQ (even greater than dust devils). She'll also drop her strong posion, which can just be dipped on a dagger and buy old school rs gold spear. Her sword is not a drop.