This bonus is multiplicative, similar to critical chance mods ... to get some Ability Strength to scale the weak point damage multiplier. Weak point multipliers are fairly rare in Warframe ...

You can get Void Relics (and resultingly, a chance to get Lavos Prime Relics) from all kinds of missions in Warframe. But, of course, there are optimized farming spots for each tier of Relics where ...

but only with the right combination of augment mods, arcanes, and Helminth abilities. Note that being in the B-tier on this tier list doesn't make a Warframe bad, their difference with A-tier ...