A paladin can absolutely solo someone in 4-5 seconds, they have ridiculous burst if you know how to use it. The downside is they use up a lot of cooldowns in the process and then basically hit like a wet noodle for 90-150 seconds (until all their cooldowns come back). They have 5 off-cooldown moves (full swing, spirits within, circle of scorn, glory slash, mercy stroke) plus fight or flight (+30% attack buff) and can also use a x-pot of STR (+20% STR). If you were at something like 90% hp and they used everything they had, you are going to die unless you purify that full swing at the right time, get a heal, or use second wind/recouperate. This is without factoring in any buffs from nodes in slaughter, if you do that it's even more crazy.

If you recognize the paladin burst you can counter it appropriately, and the paladin is just left there flapping in the wind. If you ever get a chance to play wolves den as a healer, you'll get a lot more experience both performing bursts and recognizing when a burst is coming, the meta gets pretty interesting.

I personally know at least a couple paladins on flames capable of pulling this off, and I'm guessing you probably ran into one of them.