first of all I want to apologize for my really bad english. Its not my native language and my school english was never toooo good, but I hope you can understand what I try to say.
I DM a round of 4 Players. When we played last time the players (Lvl 3) attacked two Ksariks (CR 4). They had a really hard time with them (I think next time i take LvL 3 enemies for a LvL 3 group, my bad). So one Player (Android Agent) wanted to try something different. He wanted to "climb" the Ksarik and hit it with his dagger, while riding it. The Ksarik was at the moment attacking the Vesk Soldier and was standing with its back towards the Android Agent.
I told the Agent Player, that it is possible, but hard to do so. I said he would get a +12 mali on an attack to succesfulli climb the Ksarik. (Ksarik Armorclass is 18, so he would need a 30).
He didnt want to do it.
After the round we had a really (!) long discussion. He argued that it shouldnt be that hard to climb the ksarik, because it is a big creature, and he could kindoff rodeo it like a horse or bull.
I told him that bulls and horses dont have tentacles and try to eat you while you ride them and in a rode you dont climb onto the horse/bull, you sit on it from the beginning and just try not to fall down.
He argued that he had an dagger, which he could use to climb the ksarik.
I argued that the ksarik is still armored (thick armorlike skin i think) and moves while fighting and not standing still.
I am really new to Starfinder and never played Pathfinder or DnD before (I am more a Shadowrun, Fallout or DSA Player/DM), so was the +12 to much?