Risk has rules about dice usage that are hard to recognize if you haven't played the physical board game. It's been a long time, but I believe it goes something like this: If Bulma attacks with 3 or more troops, she rolls 3 dice. If she attacks with 2, then she rolls 2 dice. If you are defending with 2 or more troops, you roll 2 dice. If you are defending with 1, you roll one dice. Each sides dice are now compared. Each roll she had that is higher than one of your rolls kill one of your troops. So if she rolls 5, 4, and 3 and you roll a 4 and a 3, her 5 will kill your 4 and her 4 will kill your 3. In this case she takes your territory without losing any troops. In the case that your rolls tie or beat hers, her troops will be killed. This means the attacker always has a good chance of losing nothing when attacking poorly defended territories. It's always good to have more troops defending choke points or critical territories and it's always good to attack poorly defended (only 1 troops) territories.
In order to beat Bulma in one vs one (more players complicates things a bit), choose all territories that give you the most troops. Money doesn't matter as much in one vs one. Always be on the attack to secure more territory that provide troops, but make sure to defend the high yield (2 or 3+ troops) territories so that you have more starting soldiers each turn. Once you control soldier production, you will win.
Also, going first is a pretty big advantage in 1vs1. If it's tough to win, consider only playing a match where you win the initial roll.