It's... not the worst I've seen?

Retribution and Rush, while fun, aren't doing you any favors. I'd toss them. Likewise, Fast Deflection is a waste. In place of those three, I see you have a Flow you aren't using for some reason. I'd put that in. I'd also work on maxing some of those mods that you have, like your Intensify, Constitution, and Stretch.

I don't think an unranked Redirection is doing you many favors. I'd rank it up, or pull it out for something more useful, like Streamline or Continuity (or both!).

That said, it's not an inexplicable build, and I certainly remember jamming in whatever mods I had handy to fill slots and capacity. Even if you stuck with the mods you have, remember that you've got 8 capacity being wasted. Maxing Intensify, Constitution, and Stretch will use of 5 of that 8 and the remaining 3 could be put into Vitality or Reflection. But at the very least, I'd dump Fast Deflection and use that polarity slot for a pumped up Vitality.

Well, when you say that you have limited mods, that already puts a limitation on how you can build your warframe, so there is not really any way you'd have a "good" build at this point.

I would say to level up the mods you do have, redirection one of the best candidates. It's never a good idea to have unranked mods on your equipment

1. Rank up your Redirection
2. Remove that rush (it's useless at rank 0) and equip Streamline (if you have it, it makes powers cheaper)
3. Rank up that Constitution because it gives a little less duration than Continuity but it's way cheaper to rank up and it gives knockdown recovery speed or something like that
4. MAYBE equip that Flow instead of that Fast Deflection (when I was a noob ^{(last week?)} I used to equip fast deflection on all my warframes, I guess that's why I think it's nice)

^{(BTW last week thing was a joke, just saying in case you didn't understand)}

Thanks for the comments, I'll see what I can do to improve :)

How do you want to play Excalibro? Are you gonna be in the fight, getting shot while doing melee? Are you gonna stay at range? With mods, the bigger question is how do you want to play or what is more fun for you. Like others have said, your build is pretty great, just with a few minor changes, like the lvling of the mods.

Build 1: Range

For a range build, pick Vitality and Steel Fiber for your health mods. These will be universal to save your rump from stray bullets. Ability mods are stretch, constitution and flow. These are great for spamming your radial blind to keep melee at range and range not shooting at you. To add some mods in, keep redirection, flow and rush. Reason for this is your at range/mid range so you can take a breather or peak around a corner and kill things. They add to survivability when you have a good amount of space to maneuver.

Build 2: Melee

Health mods are the same, but add in redirection as a constant. If you're gonna be in the enemies face, with little breathing room, you want health and shields until you get the movement system down. For abilities/utility, go with Reflex guard once you get it to max, Stretch again for Radial blind and Javelin, Intensify for added power on your powers, Constitution to help all your powers, and rush or Mobilize. Mobilize if your comfortable with the movement system or Rush if you just need a pickup moving between the mobs and your low on power for slash dash.

Overall, you want max mods for where it's important. Steel Fiber needs to be max or close to it to really take advantage of that armor against higher lvl enemies. Stretch needs to be maxed since it's your key mod for your powers that help with survivability. Rush and mobilize should be maxed, considering they are useless at low levels, but great at higher ones. Intensify should be maxed, since it helps with the DPS of all your powers and Finally, Flow should be close to maxed since it gives you more expendable energy when your not constantly picking them up.

PS: The relfex guard is really under rated. At max rank, it will auto parry range attacks while your spamming melee. It will stop the attack, counter for you, leaving a good time to hit that Radial blind or Javelin. You have most of these, just adjust your playstyle and keep in mind where you want to be most of the time in a fight. I would suggest range if you want to level your Primary or Secondary and Melee if you want to level your melee (duh lol).

You did some good things , 1 . You have added a reactor . 2 you used the exilus spot . 3 for a lack in sheild (redirection ) you used a modded fast deflection . 

By building up redirection you wont need the fast deflection , replace reflection with rush in the

exilus spot . Add flow to the empty slot . 

In general terms redirection is your life saver , it takes a shit ton of cores to build up , in early game , so if you aren't using your extra multiples , (  try to keep 1 of every mod ) it's time to fuse them to this mod . Don't mod anything else until you get it to 200% , like i said it'll take time to do . 

1 tip on gameplay , if the blue number reaches zero        RUN AWAY