mount feat:

DM allowed, with monstrous mount feats, to gain a nightmare.
i am lvl 13,cavalier of the sword, with spring attacks, charging feats and dazing assult.
so can build a nice mount.
it have int of 13, so - much feat combo is open. (total of 4) .
spring attack offer amazing position for me.

.
1 - light armor (free) & dodge
2 - ???
5 - free mobility + ???
8 - spring attack
10 - ???
13 - ???

option 1:
power attack (must have feat here ) > imp bull rush > grt bull rush > quick bull rush (lvl 13)
position the foe, good for damage as well (although it's not his task), quick bull rush works well on spring attacks and charges .

option 2:
expertise > imp trip > grt trip > lunge (**13)
trip is amazing, but work less and less those levels.

option 3:
expertise > imp dirty trick > grt dirty trick> quick dirty trick(**13)
quick dirty allow it to work with spring attacks. many options, but a bit weird for a horse...

option 4:
lunge > power attack > cornugon smash (free intimidate after power attack) > whatever (13)
adding shaken with good intimidate is nice free action and rider for the attack.

option 5:
iron will > toughness > reflexes > body-gaurd (13) (benevolent will be needed) .
a bit boring tank defender.

What's your own build made for? The best choice would be one that synergises with your own style

Also, whats the charaacters alignment? You may be planning fun things, but your DM might just be dragging you into a trap with the nightmare kicking you in the ass while your back is turned. It IS evil after all

the nightmare isnt evil, it was, as was i.
it lost all his abilities (fire, smoke, plane shfit) and kept the stats, basic attacks and flight.

my build is : lvl 9 cavalier emmisary of the sword. lvl 2 fighter roughrider.

1 cav1 Power attack skill focus ride mounted combat
2 cav2 2+ sv - will
3 cav3 monstrous mount1
4 cav4
5 cav5 ride by attack+ mobility
6 cav6 Wheeling Charge (free)
7 cav7 dodge
8 cav8 skill focus intim (free)+spirited charge
9 cav9 monstrous mount2+trick riding
10 fgr1 heavy armor+spring attack
11 fgr2 boon companion+dazing assault
12 cav10
13 cav11 Furious focus

tactics is spring attack in&out, position and charge.
when no movement is possible - attack and daze.
when main fighter falls, i cover his tank,

So....advice?

The tripping path seems to add most with your build. The size and quadrupedity of the nightmare adding in a thing or two with the downside being what tou already know + a still nonzero risk of getting tripped and sending you both onto your backs.
If you can expect some dextrois things to hit you might also think about some kind of feint.

Also dont forget your tactician feature for free teamwork feats with a swift act

If he uses a weapon with the Trip property this at least eliminates the risk of rider and mount falling over inside a Giant's threatened zone.

If he uses a weapon with the Trip property this at least eliminates the risk of rider and mount falling over inside a Giant's threatened zone.

Nightmares dont do weapons....

The tripping path seems to add most with your build. The size and quadrupedity of the nightmare adding in a thing or two with the downside being what tou already know + a still nonzero risk of getting tripped and sending you both onto your backs.

If you can expect some dextrois things to hit you might also think about some kind of feint.

Also dont forget your tactician feature for free teamwork feats with a swift act

I agree trip ( with greater trip) is the nastiest effect, as foe is stuck and provokes.

But... Trip can be usded with spring attack without aoo , even withot feats.
So, perhaps bulls or dirty are adding more?.

So,
Power attack and bull rush offer the most options, as power is always usable and bulls, with the provoke of aoo is at least a decent useage.

Trip is indeed lovely, but if foe is unteippable, the mount will be kinna stuck.

Dirty trick is costly, but offer something rwally nice that might work with 50% of the times for the mount.