I've tried all MCs once - it was terrible against shields. I've settled with bursts when they seemed like a reasonable compromise, but stuff got reworked so often - I mostly lost track what is efficient. It's mostly about being too lazy to reload so often.
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Shields have good resist against kinetic, high resist against explosive and negative resist against thermal.
Regular Hull/armour has high resist against thermal and low/negative resist against kinetic and explosive.
Reflective and reactive armour, which exist since beta times, can significally change a hulls resistance.
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All lasers deal thermal damage, cannons and multicannons deal kinetic damage, missiles deal explosive damage.
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Negative resist indeed means that damage is multiplied from the base damage, while positive resists really cut into a weapons damage.
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It was like that during the closed beta, it's still like that now. I wouldn't even be aware of any changes for this since the closed beta. Only when looking at the Railgun and Plasma Accelerator, things get a bit different, they deliver a damage mix and Plasma Accelerators a while ago also were changed to deliver part of their damage as "raw" damage.
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I guess those things above all by themselves already explain why your all-multicannon setup felt rather weak against shields. MCs have very high damage per second, but the kinetic resist which all shields have shows some effect. They really shine when shields go down, though.
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Mind you, the above guidelines are not true any more when engineering was done to a degree which players (but not NPC) tend to do. Smart engineering can push shields to have high resistances to all damage types. For hull/armour, it's a good deal harder to get to similarily high resists. It still is possible and players do it. And last not least, one other oddball: multicannons can be engineered to fire incendiary rounds, which turns a significant part of their damage to thermal damage. It makes them quite effective against shields, while costing them damage against hull. I only mention that for completenes sake, it has no influence on the described scenario.
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But keep in mind, getting all positive resists, preferably close to the cap, you only see for players. Based on my experience, i would say that NPCs at best get their weaknesses close to single digit negatives, but never to zero and definitely not above. So against an NPCs shields, theoretical lasers have the advantage of being "boosted" by negative resists. (But Keep in mind that in many scenarios this is not enough of an advantage to compensate for the lower damage per second of laser weapons. )
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So based on everything added up, i would expect that the FLD you were fighting was built with reflective armour, perhaps with one or another HRP on board. If all your weapons were lasers, then this setup indeed was able to absorb a lot of damage, and even a bit more if some engineering was done to it. But i would guess that engineering, if there was any in that NPCs ship, had a only a small impact, while the resists made most of the difference.
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So all in all, it's never a bad idea to bring a mix of weapons. The most basic setup is lasers and multicannons. Use the lasers to bring down shields. You don't really loose much by having a few lasers less, especially on a non-engineered ship. A setup with all lasers generally is limited by the power distributor and doesn't loose any effective firepower by removing some lasers. (Except it you are jousting, of couse. ) At the same time, multicannons can take those slots, they have very low distributor draw and can really add to your damage output once the targets shields are down and kinetic damage is needed.
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