It isn't that sort of game.
Risus already has a perfectly fine rule in place for handling equipment.
If you don't have the required equipment for the task at hand, but you have enough to attempt it, you roll half as many dice as normal (round up).
Weapons and Armour are an extension of this rule. Let's look at the Warhammer 40k Risus game I'm prepping at the moment.
Most personal weapons are designed to take on enemy infantry, with armour being a real possibility As such I'd say that Imperial Guardsman (2) with Lasgun can attack the Chaos Marine (4) in Power Armour as normal. If it were a Terminator (5), I'd knock it down to half-dice.
Most personal weapons are not designed to take on vehicles. Trying to shoot at an Ork Trukk (3) with your Bolter? I'd allow half-dice. Trying to shoot an Ork Stompa (7) with the Bolter? No chance. Work out another way of taking it down.
Similarly, large numbers of opponents may require weapons that can dish out destruction on a large scale. A Hormagaunt Swarm (5) will need something with automatic fire or a blast. Frag Missiles and Autocannons will work just fine, but a Plasma Pistol is going to leave you rolling half dice. If you team up with your buddies, though, you're good to roll full dice.
Using this system, it's perfectly fine to have a Warlord Titan (8) without worrying that it's too easy to take down. Infantry weapons won't scratch it, and even heavy weapons will only give half dice. This is where you call in for Superheavy support. Roll in the Shadowsword Super Heavy Tank (5).
It's worth stressing the importance of transparency here. Don't wait until the players are rolling dice to tell them that their lasgun won't be able to penetrate tank armour. Let them know as soon as the tank appears.
Space Marines are a notable example here, and I can already feel the rage against my earlier ruling on lasguns vs power armour. The actual power level of marines varies across the history of the setting. It's best to establish, ahead of the game if they're fair targets for lasgun fire, or mostly protected.
Some GMs may even make them immune to such puny weapons. These are the times you really need to warn your players in advance.