I was wandering, what kind of tactics could a rogue trader use to attack another rogue trader, besides outright combat. When i say attack it actually means "beat into submission"-like tactics, but without expose him/herself, counting they are in por wanderer. Im having a writters-blockade.

As usual, 'depends'.

If moving openly, that could mean either political barbs in social events to reduce their standing and make them look weak, or box their ears economically-outbid them, constrict markets, and accept a pinch on your own purse for the chance to empty theirs.

Covertly, that means bribery, theft and blackmail to start with-lots of underhanded things with plausible deniability. In advanced cases, that escalates to sabotage and assassination.

If you are out of specific ideas, look up the misfortunes table in the rulebook-that covers most types of incidents that are likely to happen without open battle. Figuring out a way to inflict the Adeptus Terra on opponents and arranging mishaps for enemies should be familiar to any dynasty that survives its first venture.

Those are excellent ideas, thanks!

Orchestrating an entire scenario where the Rogue Trader wanting to give the hurt sets up seemingly innocuous situations where one can attribute to bad luck (i.e. false payment account, bogus quests, fudging what the request actually is) and all these events lead up to where the Rogue Trader getting the hurt eventually loses his warrant of trade and the other Rogue Trader is there to watch it, smiling the whole time while flaunting his new wealth/prestige.

Essentially, look at the stuff that makes a Dynasty function, and attack those:

• Their profit score - this is literal financial holdings (subject to robbery or creative accounting to collapse them) and shares in commercial or industrial ventures (subject to industrial espionage and sabotage)
• Their ability to acquire and keep allies - without access to support from the Adeptus Mechanicus, Adeptus Telepathica Astra and Navis Nobilite, any interstellar entity grinds to a halt pretty quickly (subject to undercutting them for treaties) and the individuals within the crew represent the Rogue Trader's voice within said organisation (subject to character - and actual - assassination)
• Their reputation - a lot of imperial organisations, like the Inquisition and the Navy, are predisposed to assume any Rogue Trader is engaged in illegal and heretical dealings until proven otherwise, and even then, some of them just assume that's because the Rogue Trader hid the evidence well (subject to framing them for pirate attacks and raids on colony worlds, or involvement in the 'cold trade' of xenos artefacts)
• Their ship - shooting it with macrobatteries is simple enough, but sabotage can be just as deadly, and so can introducing things like heretical (or even genestealer) cults. After all, a common enough technique (so much so that there are specific rules for it) to replenish crew is to draft a world's prison population. Suppose they were to take on some rather unpleasant heretics at the time without knowing it?
• Their overconfidence - "Look! an ancient map showing a route through the Excubitores Asteroid Belt to a hidden archeotech vault!" "You don't suppose this is just a massive trap, a forgery leading us through a largely unsafe route into a carefully prepared minefield, do you?" "Really? Who would do a thing like that?"