Burning Convictions achievement how, when a siege battle is forcibly ended?

I've tried multiple times to get this achievement in which you destroy 75% of a settlement in a siege battle. I've brought in Onagers and flaming archers and tried repeatedly with multiple enemy settlements, but after a certain point, the game will forcibly end the battle before I'm done and I don't get the achievement.what should I do? Or is this the sort of achievement I can only get besieging a tiny village after having it under siege for 8 years so it's already half-destroyed by the time I engage it in battle?

Nah, you need to do that 75% damage during battle. Once sieged one city for an eternity, settlement damage was huge, but did not get achievement.
Forcibly ends, so you mean enemy routs and you win via morale drop? Its very likely that enemy garrison routs before you get enough damage. Never bothered with such a achievement, but you could try to make enough damage to town, without fighting enemy garrison.
Try to use raider cavalry. Best bet would avoid garrison while raiders sweep inside city and start torch throwing. To get garrison out of town, they need to have balance of power in their side, or hire an onager and AI will go :wub: crazy and try destroy your onager.

so i'd basically need to instantly besiege a settlement with onagers and such without waiting a turn?And no, it doesn't forcibly end there. When the enemy routs and it gives me the End Battle option in the center of the screen, but I can keep going, I keep going. After like 5 minutes with that, it just forces me to stop by slamming the Victory square on me.There's really no way for me to be able to set enough of the settlement on fire without the enemy garrison breaking and routing. they always end up either slaughtering my archers (I'm playing as the Huns and using Fall of the Eagles mod, and only ground archers have flaming arrows ability) or breaking and running.

Yeah, I guess. Or wait one turn to see if enemy attacks, though AI garrison need to be stonger than your attacking force. Add to that huns morale penalty against christians and it can be over in seconds. Its a race against time lol.
Still, roman towns are much easier to burn down, since they are build thickly. And since they have heavy infantry, raider cavalry would be ideal.
Best bet would be simply play vanilla and get that achievement, then turn back fall of eagles and forget vanilla.

Its easy to get this (and any other achievement that requires a battle ie: kill 1000 men with one unit etc):

Create a custom battle (doesnt have to be in-campaign), select a small undefended town map, surround the city with your general and x2 very heavy cavalry units and fill all the other army slots with heavy onagers or similar. For the enemy army AI place only 1 general unit and 1 very light SWORD (not pikes/spears) unit. Start the battle, remove the fire at will option from the heavy onagers so that they dont waste ammo on the enemy troops and slowly/fastly start firing on the town buildings, the moment they catch on fire redirect your onager unit's fire on another non-burning building. Keep your non-siege weapon/onager units (general and cavalry) away from the enemy units unless they come for your siege weapons until u have the town burning/ed and then attack them, kill them, win and get the achievement.

Similarly for other such achievements, fil the enemy army with the needed very light sword infantry units that fill up 1000 enemy troops in total, place a general and 1 (or 2) very heavy infantry spear unit keep your general safe and away from the action but near enough to buff ur troops, slaughter, win, get chevo.

for naval battles u can do the same.